Require Python 3.10 and use the annotations it provides

[birkholz-cubert]

What

• Raises requires-python from >=3.9 to >=3.10, drops the typing_extensions dependency and the 3.9 classifier, and moves ruff to target-version = "py310".
• Removes the try/except ImportError fallback in cuvis/doc.py; ParamSpec and TypeAlias are in typing from 3.10.
• Rewrites 71 annotations: Union[A, B] and Optional[A] to A | B and A | None, and Tuple, FrozenSet, Sequence, Callable, Awaitable to builtins and collections.abc.
• Fixes register_ready_callback, whose callback parameter was annotated Callable[None, Awaitable[None]].

Why

• Python 3.9 reached end of life in October 2025, and it was the only reason for the typing_extensions dependency and the import fallback. CI already runs 3.11 alone, so no workflow changes.
• The annotation rewrite is ALL-1431, which was blocked on the floor.
• Callable[None, ...] was never a valid Callable form. typing.Callable did not validate it; collections.abc.Callable does and raises TypeError at import, so the migration surfaced it. The implementation calls callback() with no arguments, so Callable[[], Awaitable[None]] is what it always meant.

Pitfalls

• Breaking for anyone still on 3.9. That is the point, but it is a floor bump in a wrapper-only revision, so it wants a deliberate version decision before release.
• The annotation sweep touches 15 modules and is nearly all diff noise. Reviewing it commit-by-file is easier than reading the whole diff; it was applied with ruff check --select UP006,UP007,UP035,UP045 --fix, not by hand.
• ImageData.__getitem__ deliberately keeps Union. Its member list contains a forward reference, and | cannot join a type to a string at runtime. Ruff declines to rewrite it for the same reason.

Verification

• 123 passed against the installed SDK on 3.11.
• Annotations evaluate cleanly on 3.10, 3.12 and 3.14; 3.9 now fails at import as intended.
• ruff check, ruff format --check and scripts/check_changelog.py clean.