[3.13] gh-118789: Restore hidden `_PyWeakref_ClearRef` (GH-118797) (GH-118903)

gh-118789: Restore hidden `_PyWeakref_ClearRef` (GH-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.
(cherry picked from commit db5af7da092409030c9fbe0a3a986bd0ee441b8b)

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