Fix __hash__ type annotation

[aaron-skydio]

Annotating the type of this as None like is currently done causes mypy to generate an error, like this:

hash.py:2: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[object], int]")

python/mypy#4266

Annotating this as ClassVar[Any] seems to work. My other attempt of annotating as e.g. Optional[Callable] doesn't

[auscompgeek]

This follows the convention used by typeshed to mark a class as unhashable. e.g. array.array

[aaron-skydio]

Agree - however, note the # type: ignore[assignment] there that's also required for this to pass; otherwise mypy emits the error I mentioned in the PR description. pybind11-stubgen doesn't currently have a way to emit comments like that, and adding that would be a more involved change, which is why I went with this option. If this isn't acceptable though and we need to emit the comment instead I understand

[skarndev]

According to this mypy issue (python/mypy#4266) mypy is automatically treating types with __eq__ defined as those having __hash__ set to None. That does not cover the case when the user wants to explicitly mark the type as unhashable even when it does not define __eq__. The case is pretty exotic, can someone provide some reasoning for doing so when binding C++? Perhaps something to do with virtual and trampoline objects?