Dunder methods should allow overriding to None

[JelleZijlstra]

This should be legal:

class A:
    def __hash__(self) -> int: ...
class B(A):
    __hash__ = None

See here for more python/typeshed#2148.

[emmatyping]

Isn't this essentially a duplicate of #4266?

[ilevkivskyi]

Yes, I think this is a duplicate.