Ensure data read from enum signals is picklable

[DominicOram]

For separation of RE and callbacks (e.g. using zmq) data from a read needs to be picklable. Currently if I do:

import pickle
from enum import Enum

import bluesky.plan_stubs as bps
from bluesky.run_engine import RunEngine
from ophyd_async.core import DeviceCollector, StandardReadable
from ophyd_async.epics.signal import epics_signal_r


class SynchrotronMode(str, Enum):
    SHUTDOWN = "Shutdown"
    INJECTION = "Injection"
    NOBEAM = "No Beam"
    DEV = "Mach. Dev."
    USER = "User"
    SPECIAL = "Special"
    STARTUP = "BL Startup"
    UNKNOWN = "Unknown"


class Device(StandardReadable):
    def __init__(
        self,
    ):
        self.mode = epics_signal_r(SynchrotronMode, "CS-CS-MSTAT-01:MODE")


RE = RunEngine()

with DeviceCollector():
    my_device = Device()


def my_plan():
    test = yield from bps.rd(my_device.mode)
    pickled_string = pickle.dumps(test)
    print(test, pickled_string)


RE(my_plan())

I get:

Traceback (most recent call last):
  File "/scratch/ffv81422/hyperion/dodal/scratch.py", line 40, in <module>
    RE(my_plan())
  File "/scratch/ffv81422/hyperion/dodal/.venv/lib/python3.10/site-packages/bluesky/run_engine.py", line 918, in __call__
    plan_return = self._resume_task(init_func=_build_task)
  File "/scratch/ffv81422/hyperion/dodal/.venv/lib/python3.10/site-packages/bluesky/run_engine.py", line 1057, in _resume_task
    raise exc
  File "/scratch/ffv81422/hyperion/dodal/.venv/lib/python3.10/site-packages/bluesky/run_engine.py", line 1687, in _run
    raise err
  File "/scratch/ffv81422/hyperion/dodal/.venv/lib/python3.10/site-packages/bluesky/run_engine.py", line 1547, in _run
    msg = self._plan_stack[-1].send(resp)
  File "/scratch/ffv81422/hyperion/dodal/scratch.py", line 36, in my_plan
    pickled_string = pickle.dumps(test)
_pickle.PicklingError: Can't pickle <enum 'GeneratedChoices'>: attribute lookup GeneratedChoices on ophyd_async.epics._backend.common failed

Acceptance Criteria

• I can pickle the readback from an Enum signal
• There are tests covering this

[coretl]

Fixed by #237