Remove un-needed __hash__ methods from stdlib

[sobolevn]

While working on stubs for django I've noticed that sometimes people redefine __hash__.

But, this is not what we do for other methods. For example, if base class A defines foo, its child class B does not need to also define foo. Parent's definition will be used by default. We only annotate redefinitions.

So, __hash__ is defined in object. Child classes may not redefine it, unless:

• __hash__ returns something different, not int (like enum.Enum does right now)
• __hash__ is set to be ClassVar[None]
• __hash__ is reset after parent's ClassVar[None] to indicate that subclass is immutable again
• __hash__ is required to be set explicitly as a part of @abstractmethod parent's def, like numbers.Number does

Mypy sample: https://mypy-play.net/?mypy=latest&python=3.10&gist=cef2a15be8adfa8d646d260232e6b38a

from typing import Hashable

class MyHash:
    def __hash__(self) -> int:
        ...

class MyClass: ...

def requires_hash(o: Hashable): ...

requires_hash(MyHash())   # ok
requires_hash(MyClass())  # ok

What do others think?

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

LGTM, although I'd like another maintainer to confirm.

[AlexWaygood]

LGTM too, but I'd like to get @JelleZijlstra's feedback. I think he had some Thoughts when I tried to do something along these lines a while back with some other methods.

[sobolevn]

@JelleZijlstra frienly ping 🙂