[3.13] gh-117657: Fix `__slots__` thread safety in free-threaded build (GH-119368) (#120655)

Fix a race in `PyMember_GetOne` and `PyMember_SetOne` for `Py_T_OBJECT_EX`.
These functions implement `__slots__` accesses for Python objects.
(cherry picked from commit 362cd2680b45a36c3467b9721ff7fc0ceb338452)

Co-authored-by: Daniele Parmeggiani <8658291+dpdani@users.noreply.github.com>