Make None compatible with Hashable

[JukkaL]

Fixes #8768.

[erictraut]

We received a similar feature request for pyright. I considered doing a spot fix specifically for __hash__, but this struck me as a bit hacky and not very generalizable, since there are other valid protocols that could also apply to None.

I thought of another approach that might work better. Could we define a NoneType in typing.pyi that defines the methods supported by None? Type checkers could then use this class definition when doing protocol validation. Interested in your thoughts.

If you're open to that approach, I'd be happy to submit a PR to typeshed that defines NoneType.

[erictraut]

@JukkaL do you have any thoughts on my proposal above?

[JukkaL]

It sounds reasonable, since the type of None is not available in the builtins or types in Python 3. I don't see a use case for making it available at runtime, however. It was removed from types in Python 3 (types.NoneType exists in Python 2.7), and I'd assume this happened for a reason. So maybe we should be name it _NoneType to make it more clear that it's an internal definition that's only supposed to be used by tools?

[erictraut]

Thanks, naming it _NoneType is a great suggestion. I'll submit a PR to typeshed.