Emit PEP 585 syntax from stubgen

[ktbarrett]

Feature

stubgen currently does not emit PEP 585 syntax, it should, it is permissible, and typeshed has been using PEP 585 syntax for a long while now.

For example, stubgen outputs the following non-PEP-585 syntax in the most current version.

from typing import Tuple

def get_sim_time() -> Tuple[int, int]: ...

Pitch

stubgen should output PEP-585 syntax.

def get_sim_time() -> tuple[int, int]: ...

[hamdanal]

I have a working prototype of this that I haven't submitted yet. It uses some of the changes made in #16519 so I would like for that PR to get merged first (the other PR is also less controversial so if it is not accepted I don't this will be accepted).