Type setup method

[Avasam]

Summary of changes

Step 6.4 of https://github.com/pypa/setuptools/issues/2345#issuecomment-1627561833

Once https://peps.python.org/pep-0728/ lands, this can likely be simplified by exposing and combining a TypedDict from _distutils to use as an annotation for **attrs. I don't think we need to wait for it, implementation can be simplified later, I'm just keeping track here:

• 2024/10/22: https://github.com/microsoft/pyright/releases/tag/1.1.386 released with updated experimental implementation
• 2025/03/25: https://github.com/python/typing_extensions/releases/tag/4.13.0 released with updated backport implementation
• 2025/08/15: PEP 728 has been approved
• 2025/08/21 https://github.com/microsoft/pyright/releases/tag/1.1.404 released marking PEP 728 support as no-longer experimental
• Mypy is still missing implementation: https://github.com/python/mypy/issues/18176

Pull Request Checklist

• [ ] Changes have tests
• [ ] News fragment added in newsfragments/. (See documentation for details)