Declare support for Python 3.13

[AlexWaygood]

No description provided.

[JelleZijlstra]

  + /home/runner/work/flake8-pyi/flake8-pyi/pyi.py:1484: DeprecationWarning: Tuple.__init__ missing 1 required positional argument: 'ctx'. This will become an error in Python 3.15.

This will go away in the next beta. We could just add ctx=ast.Load() though to make things work now.

[AlexWaygood]

I think I'm inclined to keep our code how it is and wait for the next beta (scheduled to arrive in just over two weeks, and I think Thomas might be planning to expedite it even more due to some bugfixes that didn't quite make it into beta1). I'll just keep this as a draft for now.

[AlexWaygood]

Well, the latest update is that Python 3.13.0b2 is now available on setup-python, but the build they provide is apparently utterly broken:

• Python 3.12.4: Extract downloaded archive; gzip: stdin: not in gzip format actions/setup-python#886

(The issue title says "3.12", but the discussion in the issue notes that the problem occurs for 3.13.0b2 as well.)

[AlexWaygood]

Tests all pass locally for me with 3.13.0b2, though, so I guess I'll mark as "ready for review"

[srittau]

LGTM, once CI passes.

[github-actions]

This change has no effect on typeshed. 🤖🎉