Add typing_extensions.NamedTuple

[AlexWaygood]

Fixes python/typing_extensions#56

[github-actions]

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

[JelleZijlstra]

Would anything break if we unconditionally did from typing import NamedTuple as NamedTuple? We already do that for overload too.

[AlexWaygood]

Would anything break if we unconditionally did from typing import NamedTuple as NamedTuple? We already do that for overload too.

My thinking was that type checkers might want to distinguish typing_extensions.NamedTuple from typing.NamedTuple, so that they can emit an error if a user tries to create a generic typing.NamedTuple on <3.11, since that will fail at runtime. I.e., it would be a similar argument to the argument made in #7130 for declaring typing_extensions.Protocol as a distinct symbol to typing.Protocol.

This is purely hypothetical, however.