Fix TypedDict closed argument version check

[vetrovk]

Summary

• Reject the closed argument on typing.TypedDict when targeting Python versions below 3.15.
• Preserve support through typing_extensions.TypedDict on older Python versions.
• Cover both class and functional TypedDict syntax.

Details

PEP 728 adds the closed argument to the standard-library TypedDict in Python 3.15. Mypy previously accepted the argument for earlier target versions and applied closed TypedDict semantics without reporting that the runtime API was unavailable.

This change adds a semantic version check specifically for typing.TypedDict. Both closed=True and closed=False are rejected below Python 3.15, while the backport in typing_extensions remains supported.

Tests

• python runtests.py check-typeddict
• python -m pytest -n0 mypy/test/testcheck.py -k TypedDict
• python runtests.py lint

Fixes #21747

AI-assisted: tooling was used during investigation and implementation; the contributor reviewed and validated the final patch.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

altair (https://github.com/vega/altair)
+ altair/vegalite/v6/schema/_config.py:6593: error: "closed" argument to TypedDict is only available in Python 3.15 and later  [misc]
+ altair/vegalite/v6/schema/_config.py:6618: error: "closed" argument to TypedDict is only available in Python 3.15 and later  [misc]