gh-118789: Restore hidden `_PyWeakref_ClearRef` (#118797)

_PyWeakref_ClearRef was previously exposed in the public C-API, although
it begins with an underscore and is not documented. It's used by a few
C-API extensions. There is currently no alternative public API that can
replace its use.

_PyWeakref_ClearWeakRefsExceptCallbacks is the only thread-safe way to
use _PyWeakref_ClearRef in the free-threaded build. This exposes the C
symbol, but does not make the API public.

Include/cpython/weakrefobject.h

@@ -40,6 +40,8 @@ struct _PyWeakReference {
 #endif
 };
 
+PyAPI_FUNC(void) _PyWeakref_ClearRef(PyWeakReference *self);
+
 Py_DEPRECATED(3.13) static inline PyObject* PyWeakref_GET_OBJECT(PyObject *ref_obj)
 {
     PyWeakReference *ref;

Include/internal/pycore_weakref.h

@@ -111,8 +111,6 @@ extern Py_ssize_t _PyWeakref_GetWeakrefCount(PyObject *obj);
 // intact.
 extern void _PyWeakref_ClearWeakRefsExceptCallbacks(PyObject *obj);
 
-extern void _PyWeakref_ClearRef(PyWeakReference *self);
-
 PyAPI_FUNC(int) _PyWeakref_IsDead(PyObject *weakref);
 
 #ifdef __cplusplus

Misc/NEWS.d/next/C API/2024-05-08-20-13-00.gh-issue-118789.m88uUa.rst

@@ -0,0 +1,2 @@
+Restore ``_PyWeakref_ClearRef`` that was previously removed in Python 3.13
+alpha 1.