Convert typing declarations to use TypeAlias

[Dr-Irv]

See discussion here: #341 (comment)

Should we change _typing.pyi to make all these types that we create be declared with TypeAlias (imported from typing_extensions) ?

@twoertwein - your opinion?

[AlexWaygood]

Fun fact: typeshed pretends that typing_extensions is part of the standard library, meaning that all type checkers also believe that typing_extensions is part of the standard library. This means that (since this is a stubs-only project, and will never be executed at runtime) you should be able to use types from typing_extensions (and also _typeshed, which we treat similarly) without adding a dependency on typing_extensions :)

[twoertwein]

Should we change _typing.pyi to make all these types that we create be declared with TypeAlias (imported from typing_extensions) ?

I think flake8-pyi recommends that and checks for it (disable in our config for now). Definitely good to make it more explicit.

[twoertwein]

This means that (since this is a stubs-only project, and will never be executed at runtime) you should be able to use types from typing_extensions (and also _typeshed, which we treat similarly) without adding a dependency on typing_extensions :)

Could also start using _typeshed.Self in the stubs (Y019) @Dr-Irv

[Dr-Irv]

Could also start using _typeshed.Self in the stubs (Y019)

Or from typing_extensions, since Self is part of python 3.11

https://docs.python.org/3.11/library/typing.html#typing.Self

[AlexWaygood]

Could also start using _typeshed.Self in the stubs (Y019)

Or from typing_extensions, since Self is part of python 3.11

https://docs.python.org/3.11/library/typing.html#typing.Self

typing_extensions.Self is unfortunately not yet supported by mypy or pytype (pyright supports it, not sure about pyre). But _typeshed.Self is just a common-or-garden TypeVar with a special name (Self = TypeVar("Self")), so it works fine with all type checkers.