Summary of changes

Step 6.4 of #2345 (comment)

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: Add support for TypedDict extra_items= and closed= (PEP 728) python/mypy#18176

Pull Request Checklist

• Changes have tests
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