Handle PEP 604 unions inside type annotations

[vitalivo]

Changes

Use the union checker for PEP 604 unions inside type[...], as already done for typing.Union.

For example, check_type(list, type[str | Any]) currently raises TypeCheckError, even though Any permits the class. Invalid classes also lose the per-member diagnostics. The regression tests cover both positions of Any, accepted classes, and rejection messages for both union spellings.

Validation

• Python 3.12: 540 passed, 9 skipped, 9 xfailed.
• Three regression cases fail with the original checker.
• Ruff 0.15.20 lint and formatting checks pass for both changed Python files.
• Mypy 2.1.0 reports the same existing missing typeshed.stdlib.types import in _decorators.py:22 on the base commit and this patch.

Checklist

• Added tests that fail without the patch.
• Added a changelog entry.
• Documentation: no new API or syntax; this fixes support for the existing union syntax.

[coveralls]

coverage: 94.853%. remained the same — vitalivo:fix/pep604-class-unions into agronholm:master