gh-101561: Add typing.override decorator

[stroxler]

The code is pulled almost unchanged from typing_extensions:

• decorator implementation
• tests

Question that came up was where to put the type variable and what to name it:

• In typing_extensions the type variables use underscore prefixes to indicate that they are not public, but typing doesn't appear to use that convention so I named the type variable F.
• I couldn't put the type variable next to all of the other ones (used in generic container definitions) because it relies on Callable which is defined below. If this is a problem I could correct it using a forward reference, or we could try to rearrange the module to put special forms above type variables.

Testing the code:

First set up the repo, following instructions
at https://devguide.python.org/ by running:

./configure --with-pydebug && make -j

Then run the typing tests:

./python -m test test_typing -v

I ran the full test suite with

./python -m test -j3

and it came back clean except for a test_grp failure which I seem to get on trunk as well - likely something in my build is misconfigured but I'm pretty sure it is unrelated to the changes here.

• Issue: Add typing.override decorator #101561

Make sure the docs build:

cd Doc
make venv
make html

[bedevere-bot]

Most changes to Python require a NEWS entry.

Please add it using the blurb_it web app or the blurb command-line tool.

[bedevere-bot]

Most changes to Python require a NEWS entry.

Please add it using the blurb_it web app or the blurb command-line tool.

[AlexWaygood]

Thanks! Haven't looked in depth yet at all, but we'll also need:

• A news entry, as @bedevere-bot says
• An entry in Doc/whatsnew/3.12.rst
• (optionally) you can add your name to Misc/ACKS

[Fidget-Spinner]

The implementation for override LGTM.

[stroxler]

(Rebase on main)

@gvanrossum I think this is waiting on review from you

[JelleZijlstra]

Sorry for dropping this; I have a few nits but I'm happy to merge the PR. I'll leave it open for a few days in case Guido does want to take a look himself.

[gvanrossum]

I defer to @JelleZijlstra if that's okay.

[sobolevn]

Sorry for being late with a review, I've opened a follow-up for this: #102324

Thanks a lot for adding this decorator! Great work, it would be very useful! 👍