[dataclasses] Fix MISSING and KW_ONLY for Python 3.15+

[caseneuve]

In Python 3.15, dataclasses.MISSING and dataclasses.KW_ONLY were changed from custom class instances (_MISSING_TYPE, _KW_ONLY_TYPE) to builtins.sentinel objects (see: python/cpython#149086) and _MISSING_TYPE no longer exists at runtime.

This PR:

• Gates MISSING and KW_ONLY behind sys.version_info >= (3, 15) using builtins.sentinel
• Updates Field attribute types (default, default_factory, kw_only) for >= 3.15
• Adds new field() overloads for >= 3.15 using sentinel instead of Literal[_MISSING_TYPE.MISSING]
• Removes the now-stale allowlist entries from py315.txt

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

hm this might regress functionality, can't we already do MISSING = sentinel("MISSING")?

[caseneuve]

Thanks, @JelleZijlstra, that makes perfect sense. This was my first contribution, inspired by the EuroPython sprints, where the dataclasses stubs were listed as candidates for a fix.

It looks however like the correct fix is currently blocked by a tooling constraint: typeshed pins mypy to 2.3.0, which does not yet support sentinel values as types. That work is pending in python/mypy#21647.

Should we close this PR for now and revisit it once the mypy PR is merged and typeshed updates its pin, or would you prefer to keep it open as a draft until then?

[edgarrmondragon]

@caseneuve can you check if python/mypy#21647 results in no errors for your changes? MISSING: Final[sentinel] might just not work, but I'm curious about MISSING = sentinel("MISSING").

[JelleZijlstra]

This is worse than before, we should hint the specific sentinel not the sentinel type.

[JelleZijlstra]

The next mypy release will have sentinels support, so maybe let's just wait for that.