Support python 3.10 optional syntax

[peku33]

Python 3.10 introduced str | None syntax which should be equivalent to Optional[str]. At the moment such syntax isn't properly recognized by pure_protobuf.

Following declaration will raise error:

@dataclass(kw_only=True)
class Bar(BaseMessage):
    foo: Annotated[str | None, Field(1)]

with

UnsupportedAnnotationError: type annotation `str | None` is not supported

This is because get_origin in pure_protobuf.helpers._typing.extract_optional checks for Union only, while this syntax has UnionType from types origin. The rest is the same.

This probably could be easily fixed by changing is comparison to in (Union, UnionType), however special care must be taken, because (same as NoneType) UnionType was introduced in 3.10.

Thank you.

[eigenein]

Thank you for pointing it out! I've faced a very similar issue in another project, and you are correct that checking for UnionType is needed. I'll have a look; it should not be difficult to fix.