gh-118789: Add `PyUnstable_Object_ClearWeakRefsExceptCallbacks`

[colesbury]

This exposes _PyWeakref_ClearWeakRefsExceptCallbacks 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.

• Issue: gh-118789

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