Upgrade Cython to version 3

While looking at #333, I hypothesized that upgrading Cython might solve the
issue. It didn't. But upgrading should still happen at some point. This is my
work in progress on that. The tests pass, but there are two main things missing:

Problem (1):
Starting with Cython 3, you can only do `except+` or `except
+reraise_kj_exception` on `extern` functions coming from C++. (This makes sense,
and the way things were declared in Pycapnp wasn't too good.) As a result, I had
to remove a lot of these declaration. This results in some segmentation faults,
because Cython no longer detects C++ exceptions and converts them to Python
exceptions in some places.

To solve this, all `extern` declarations in `.pxd` files have to be examined and
`except +reraise_kj_exception` clauses need to be added to anything that might
throw. Previously, this was done really inconsistently. The lazy solution would
be to just add the clause everywhere, but I'm not sure what the performance
implications are.

Problem (2):
The compilation output of `python setup.py build_ext --inplace` is now full of messages like these:
```
capnp/lib/capnp.cpp: In function ‘PyObject* __pyx_f_5capnp_3lib_5capnp_18_DynamicListReader__get(__pyx_obj_5capnp_3lib_5capnp__DynamicListReader*, int64_t, int)’:
capnp/lib/capnp.cpp:4871:51: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
 4871 |   #define __PYX_STD_MOVE_IF_SUPPORTED(x) std::move(x)
      |                                          ~~~~~~~~~^~~
capnp/lib/capnp.cpp:20944:59: note: in expansion of macro ‘__PYX_STD_MOVE_IF_SUPPORTED’
20944 |   __pyx_t_2 = __pyx_f_5capnp_3lib_5capnp_to_python_reader(__PYX_STD_MOVE_IF_SUPPORTED((( ::capnp::DynamicValue::Reader)__pyx_t_7)), __pyx_t_1); if (unlikely(!__pyx_t_2)) __PYX_ERR(0, 419, __pyx_L1_error)
      |                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
capnp/lib/capnp.cpp:4871:51: note: remove ‘std::move’ call
 4871 |   #define __PYX_STD_MOVE_IF_SUPPORTED(x) std::move(x)
      |                                          ~~~~~~~~~^~~
capnp/lib/capnp.cpp:20944:59: note: in expansion of macro ‘__PYX_STD_MOVE_IF_SUPPORTED’
20944 |   __pyx_t_2 = __pyx_f_5capnp_3lib_5capnp_to_python_reader(__PYX_STD_MOVE_IF_SUPPORTED((( ::capnp::DynamicValue::Reader)__pyx_t_7)), __pyx_t_1); if (unlikely(!__pyx_t_2)) __PYX_ERR(0, 419, __pyx_L1_error)
      |                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
```
There are to many `move` calls inserted. I'm not sure if this is a Cython issue,
or if we are somehow annotating things wrong. Might be worth asking the Cython
people.

I'm not planning on working on this further in the short term. If someone wants
to take over on this, feel free.

capnp/helpers/helpers.pxd

@@ -16,20 +16,17 @@ cdef extern from "capnp/helpers/fixMaybe.h":
 
 cdef extern from "capnp/helpers/capabilityHelper.h":
     PyPromise then(PyPromise promise, Own[PyRefCounter] func, Own[PyRefCounter] error_func)
-    DynamicCapability.Client new_client(InterfaceSchema&, PyObject *)
-    DynamicValue.Reader new_server(InterfaceSchema&, PyObject *)
-    Capability.Client server_to_client(InterfaceSchema&, PyObject *)
     PyPromise convert_to_pypromise(RemotePromise)
     PyPromise convert_to_pypromise(VoidPromise)
     VoidPromise taskToPromise(Own[PyRefCounter] coroutine, PyObject* callback)
     void init_capnp_api()
 
 cdef extern from "capnp/helpers/rpcHelper.h":
-    Capability.Client bootstrapHelper(RpcSystem&)
-    Capability.Client bootstrapHelperServer(RpcSystem&)
+    Own[Capability.Client] bootstrapHelper(RpcSystem&) except +reraise_kj_exception
+    Own[Capability.Client] bootstrapHelperServer(RpcSystem&) except +reraise_kj_exception
 
 cdef extern from "capnp/helpers/serialize.h":
-    ByteArray messageToPackedBytes(MessageBuilder &, size_t wordCount)
+    ByteArray messageToPackedBytes(MessageBuilder &, size_t wordCount) except +reraise_kj_exception
 
 cdef extern from "capnp/helpers/deserialize.h":
     Node.Reader toReader(DynamicStruct.Reader reader) except +reraise_kj_exception

capnp/helpers/rpcHelper.h

@@ -6,16 +6,16 @@
 #include "Python.h"
 #include "capabilityHelper.h"
 
-capnp::Capability::Client bootstrapHelper(capnp::RpcSystem<capnp::rpc::twoparty::SturdyRefHostId>& client) {
+kj::Own<capnp::Capability::Client> bootstrapHelper(capnp::RpcSystem<capnp::rpc::twoparty::SturdyRefHostId>& client) {
     capnp::MallocMessageBuilder hostIdMessage(8);
     auto hostId = hostIdMessage.initRoot<capnp::rpc::twoparty::SturdyRefHostId>();
     hostId.setSide(capnp::rpc::twoparty::Side::SERVER);
-    return client.bootstrap(hostId);
+    return kj::heap<capnp::Capability::Client>(client.bootstrap(hostId));
 }
 
-capnp::Capability::Client bootstrapHelperServer(capnp::RpcSystem<capnp::rpc::twoparty::SturdyRefHostId>& client) {
+kj::Own<capnp::Capability::Client> bootstrapHelperServer(capnp::RpcSystem<capnp::rpc::twoparty::SturdyRefHostId>& client) {
     capnp::MallocMessageBuilder hostIdMessage(8);
     auto hostId = hostIdMessage.initRoot<capnp::rpc::twoparty::SturdyRefHostId>();
     hostId.setSide(capnp::rpc::twoparty::Side::CLIENT);
-    return client.bootstrap(hostId);
+    return kj::heap<capnp::Capability::Client>(client.bootstrap(hostId));
 }

capnp/includes/capnp_cpp.pxd

@@ -57,26 +57,12 @@ cdef extern from "kj/async.h" namespace " ::kj":
     cdef cppclass Promise[T] nogil:
         Promise(Promise)
         Promise(T)
-        T wait(WaitScope) except +reraise_kj_exception
-        bool poll(WaitScope) except +reraise_kj_exception
-        # ForkedPromise<T> fork()
-        # Promise<T> exclusiveJoin(Promise<T>&& other)
-        # Promise[T] eagerlyEvaluate()
-        # void detach(ErrorFunc)
         String trace()
         Promise[T] attach(Own[PyRefCounter] &)
         Promise[T] attach(Own[PyRefCounter] &, Own[PyRefCounter] &)
         Promise[T] attach(Own[PyRefCounter] &, Own[PyRefCounter] &, Own[PyRefCounter] &)
         Promise[T] attach(Own[PyRefCounter] &, Own[PyRefCounter] &, Own[PyRefCounter] &, Own[PyRefCounter] &)
 
-    cdef cppclass Canceler nogil:
-        Canceler()
-        Promise[T] wrap[T](Promise[T] promise)
-        void cancel(StringPtr cancelReason)
-        void cancel(Exception& exception)
-        void release()
-        bool isEmpty()
-
 ctypedef Promise[Own[PyRefCounter]] PyPromise
 ctypedef Promise[void] VoidPromise
 
@@ -109,8 +95,8 @@ cdef extern from "kj/array.h" namespace " ::kj":
 
 cdef extern from "kj/async-io.h" namespace " ::kj":
     cdef cppclass AsyncIoStream nogil:
-        Promise[size_t] read(void*, size_t, size_t)
-        Promise[void] write(const void*, size_t)
+        Promise[size_t] read(void*, size_t, size_t) except +reraise_kj_exception
+        Promise[void] write(const void*, size_t) except +reraise_kj_exception
 
 cdef extern from "capnp/schema.capnp.h" namespace " ::capnp":
     enum TypeWhich" ::capnp::schema::Type::Which":
@@ -297,7 +283,7 @@ cdef extern from "capnp/dynamic.h" namespace " ::capnp":
             Client()
             Client(Client&)
             Client(Own[PythonInterfaceDynamicImpl])
-            Client upcast(InterfaceSchema requestedSchema)
+            Client upcast(InterfaceSchema requestedSchema) except +reraise_kj_exception
             DynamicCapability.Client castAs"castAs< ::capnp::DynamicCapability>"(InterfaceSchema)
             InterfaceSchema getSchema()
             Request newRequest(char * methodName)
@@ -447,7 +433,7 @@ cdef extern from "capnp/dynamic.h" namespace " ::capnp":
             Type getType()
 
 cdef extern from "capnp/schema-loader.h" namespace " ::capnp":
-    cdef cppclass SchemaLoader:
+    cdef cppclass SchemaLoader nogil:
         SchemaLoader()
         Schema load(Node.Reader reader) except +reraise_kj_exception
         Schema get(uint64_t id_) except +reraise_kj_exception

capnp/includes/schema_cpp.pxd

@@ -673,7 +673,7 @@ cdef extern from "capnp/message.h" namespace " ::capnp":
         Annotation.Builder getRootAnnotation'getRoot< ::capnp::schema::Annotation>'()
         Annotation.Builder initRootAnnotation'initRoot< ::capnp::schema::Annotation>'()
 
-        DynamicStruct_Builder getRootDynamicStruct'getRoot< ::capnp::DynamicStruct>'(StructSchema)
+        DynamicStruct_Builder getRootDynamicStruct'getRoot< ::capnp::DynamicStruct>'(StructSchema) except +reraise_kj_exception
         DynamicStruct_Builder initRootDynamicStruct'initRoot< ::capnp::DynamicStruct>'(StructSchema)
         void setRootDynamicStruct'setRoot< ::capnp::DynamicStruct::Reader>'(DynamicStruct.Reader)
 
@@ -696,7 +696,7 @@ cdef extern from "capnp/message.h" namespace " ::capnp":
         StructNode.Reader getRootStructNode'getRoot< ::capnp::schema::StructNode>'()
         Annotation.Reader getRootAnnotation'getRoot< ::capnp::schema::Annotation>'()
 
-        DynamicStruct.Reader getRootDynamicStruct'getRoot< ::capnp::DynamicStruct>'(StructSchema)
+        DynamicStruct.Reader getRootDynamicStruct'getRoot< ::capnp::DynamicStruct>'(StructSchema) except +reraise_kj_exception
         AnyPointer.Reader getRootAnyPointer'getRoot< ::capnp::AnyPointer>'()
 
     cdef cppclass MallocMessageBuilder(MessageBuilder) nogil:
@@ -798,7 +798,7 @@ cdef extern from "capnp/serialize.h" namespace " ::capnp":
         FlatArrayMessageReader(WordArrayPtr array, ReaderOptions) except +reraise_kj_exception
         const word* getEnd() const
 
-    void writeMessageToFd(int, MessageBuilder&) nogil except +reraise_kj_exception
+    void writeMessageToFd(int, MessageBuilder&) except +reraise_kj_exception nogil
 
     WordArray messageToFlatArray(MessageBuilder &) nogil
 
@@ -816,6 +816,6 @@ cdef extern from "capnp/serialize-packed.h" namespace " ::capnp":
         PackedFdMessageReader(int) except +reraise_kj_exception
         PackedFdMessageReader(int, ReaderOptions) except +reraise_kj_exception
 
-    void writePackedMessage(BufferedOutputStream&, MessageBuilder&) nogil except +reraise_kj_exception
-    void writePackedMessage(OutputStream&, MessageBuilder&) nogil except +reraise_kj_exception
-    void writePackedMessageToFd(int, MessageBuilder&) nogil except +reraise_kj_exception
+    void writePackedMessage(BufferedOutputStream&, MessageBuilder&) except +reraise_kj_exception nogil
+    void writePackedMessage(OutputStream&, MessageBuilder&) except +reraise_kj_exception nogil
+    void writePackedMessageToFd(int, MessageBuilder&) except +reraise_kj_exception nogil

capnp/lib/capnp.pxd

@@ -28,7 +28,7 @@ cdef class _StringArrayPtr:
     cdef StringPtr * thisptr
     cdef object parent
     cdef size_t size
-    cdef ArrayPtr[StringPtr] asArrayPtr(self) except +reraise_kj_exception
+    cdef ArrayPtr[StringPtr] asArrayPtr(self)
 
 cdef class SchemaLoader:
     cdef C_SchemaLoader * thisptr
@@ -38,7 +38,7 @@ cdef class SchemaParser:
     cdef public dict modules_by_id
     cdef list _all_imports
     cdef _StringArrayPtr _last_import_array
-    cpdef _parse_disk_file(self, displayName, diskPath, imports) except +reraise_kj_exception
+    cpdef _parse_disk_file(self, displayName, diskPath, imports)
 
 cdef class _DynamicOrphan:
     cdef C_DynamicOrphan thisptr
@@ -79,10 +79,10 @@ cdef class _DynamicStructBuilder:
     cdef _init(self, DynamicStruct_Builder other, object parent, bint isRoot=?, bint tryRegistry=?)
 
     cdef _check_write(self)
-    cpdef to_bytes(_DynamicStructBuilder self) except +reraise_kj_exception
-    cpdef to_segments(_DynamicStructBuilder self) except +reraise_kj_exception
-    cpdef _to_bytes_packed_helper(_DynamicStructBuilder self, word_count) except +reraise_kj_exception
-    cpdef to_bytes_packed(_DynamicStructBuilder self) except +reraise_kj_exception
+    cpdef to_bytes(_DynamicStructBuilder self)
+    cpdef to_segments(_DynamicStructBuilder self)
+    cpdef _to_bytes_packed_helper(_DynamicStructBuilder self, word_count)
+    cpdef to_bytes_packed(_DynamicStructBuilder self)
 
     cpdef _get(self, field)
     cpdef _set(self, field, value)
@@ -128,7 +128,7 @@ cdef class _DynamicEnum:
     cdef public object _parent
 
     cdef _init(self, capnp.DynamicEnum other, object parent)
-    cpdef _as_str(self) except +reraise_kj_exception
+    cpdef _as_str(self)
 
 cdef class _DynamicListBuilder:
     cdef C_DynamicList.Builder thisptr
@@ -146,11 +146,11 @@ cdef class _DynamicListBuilder:
 cdef class _MessageBuilder:
     cdef schema_cpp.MessageBuilder * thisptr
     cpdef init_root(self, schema)
-    cpdef get_root(self, schema) except +reraise_kj_exception
-    cpdef get_root_as_any(self) except +reraise_kj_exception
-    cpdef set_root(self, value) except +reraise_kj_exception
-    cpdef get_segments_for_output(self) except +reraise_kj_exception
-    cpdef new_orphan(self, schema) except +reraise_kj_exception
+    cpdef get_root(self, schema)
+    cpdef get_root_as_any(self)
+    cpdef set_root(self, value)
+    cpdef get_segments_for_output(self)
+    cpdef new_orphan(self, schema)
 
 cdef to_python_reader(C_DynamicValue.Reader self, object parent)
 cdef to_python_builder(C_DynamicValue.Builder self, object parent)