[3.13] gh-118789: Add `PyUnstable_Object_ClearWeakRefsNoCallbacks` (GH-118807) (...

[3.13] gh-118789: Add `PyUnstable_Object_ClearWeakRefsNoCallbacks` (GH-118807) (#120695)

This exposes `PyUnstable_Object_ClearWeakRefsNoCallbacks` as an unstable
C-API function to provide a thread-safe mechanism for clearing weakrefs
without executing callbacks.

Some C-API extensions need to clear weakrefs without calling callbacks,
such as after running finalizers like we do in subtype_dealloc.
Previously they could use `_PyWeakref_ClearRef` on each weakref, but
that's not thread-safe in the free-threaded build.

(cherry picked from commit e8752d7b80775ec2a348cd4bf38cbe26a4a07615)

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>