gh-118789: Add PyUnstable_Object_ClearWeakRefsNoCallbacks (#118807)

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.

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

Doc/c-api/weakref.rst

@@ -96,3 +96,19 @@ as much as it can.
    This iterates through the weak references for *object* and calls callbacks
    for those references which have one. It returns when all callbacks have
    been attempted.
+
+
+.. c:function:: void PyUnstable_Object_ClearWeakRefsNoCallbacks(PyObject *object)
+
+   Clears the weakrefs for *object* without calling the callbacks.
+
+   This function is called by the :c:member:`~PyTypeObject.tp_dealloc` handler
+   for types with finalizers (i.e., :meth:`~object.__del__`).  The handler for
+   those objects first calls :c:func:`PyObject_ClearWeakRefs` to clear weakrefs
+   and call their callbacks, then the finalizer, and finally this function to
+   clear any weakrefs that may have been created by the finalizer.
+
+   In most circumstances, it's more appropriate to use
+   :c:func:`PyObject_ClearWeakRefs` to clear weakrefs instead of this function.
+
+   .. versionadded:: 3.13