Add functools.partial.__get__ for Python 3.14

[apoorvdarshan]

Fixes #15974.

Python 3.14 made functools.partial a descriptor (python/cpython#121027), adding a __get__ method, but the stub was missing it.

This adds __get__ to partial, gated on sys.version_info >= (3, 14), mirroring the existing partialmethod.__get__ in the same file (-> Callable[..., _T]).

The runtime signature on 3.14 is (self, instance, owner=None, /) (positional-only), which the stub matches. When accessed on an instance the descriptor returns a bound method; on the class it returns the partial itself — both are Callable[..., _T].

python tests/stubtest_stdlib.py functools passes on 3.14 with this change (no new stubtest errors), and tests/mypy_test.py stdlib/functools.pyi succeeds.

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Disclosure: this change was prepared with the assistance of an AI tool (Claude Code). I verified the behavior against CPython 3.14, ran stubtest and the mypy test locally, and take responsibility for the contribution and will respond to review feedback personally.

[srittau]

There are failing CI checks. Please have a look at them before we can consider this PR.

[apoorvdarshan]

Thanks @srittau. Fixed the CI failure: the new functools.partial.__get__ base signature (optional owner, returns Callable[..., _T]) made boltons' InstancePartial/CachedInstancePartial __get__ overrides [override]-incompatible, since they declared obj_type as required and untyped. I updated both overrides to match the base (positional-only params, optional obj_type: type[Any] | None = None, return Callable[..., Incomplete]). mypy_test.py stubs/boltons now passes on both 3.14 and 3.15, and the stdlib change is unchanged.

[apoorvdarshan]

@srittau The previous commit resolved the mypy [override] error but broke stubtest: boltons's runtime __get__(self, obj, obj_type) requires obj_type (no default, positional-or-keyword), which no longer matched the stub after I added a default and /.

I've reverted both InstancePartial.__get__ and CachedInstancePartial.__get__ back to a signature matching the runtime (def __get__(self, obj, obj_type): ...) and added # type: ignore[override] for the LSP incompatibility that the new functools.partial.__get__ base method introduces (the base's owner is optional, but boltons requires obj_type). Also dropped the now-unused Any/Callable imports.

Verified locally: mypy_test.py stubs/boltons passes on both 3.14 and 3.15, and stubtest_third_party.py boltons now reports success.

[github-actions]

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