Fix filter_version caching bug exposed after filtering nixpkgs

mach_nix/requirements.py

@@ -189,11 +189,10 @@ def parse_reqs_line(line):
 
     return name, extras, all_specs, build, marker
 
-
 @cached(keyfunc=lambda args: hash((tuple(args[0]), args[1])))
 def filter_versions(
         versions: List[Version],
-        req: Requirement):
+        req: Requirement) -> List[Version]:
     """
     Reduces a given list of versions to contain only versions
     which are allowed according to the given specifiers
@@ -202,7 +201,7 @@ def filter_versions(
     if not req.specs:
         # We filter version with an empty specifier set, since that will filter
         # out prerelease, if there are any other releases.
-        return SpecifierSet().filter(versions)
+        return list(SpecifierSet().filter(versions)) # return a list in both cases
     matched_versions = []
     for specs in req.specs:
         matched_versions.extend(