gh-116322: Rename PyModule_ExperimentalSetGIL to PyUnstable_Module_SetGIL (GH-118645)

Python/sysmodule.c

@@ -3775,7 +3775,7 @@ _PySys_Create(PyThreadState *tstate, PyObject **sysmod_p)
         return _PyStatus_ERR("failed to create a module object");
     }
 #ifdef Py_GIL_DISABLED
-    PyModule_ExperimentalSetGIL(sysmod, Py_MOD_GIL_NOT_USED);
+    PyUnstable_Module_SetGIL(sysmod, Py_MOD_GIL_NOT_USED);
 #endif
 
     PyObject *sysdict = PyModule_GetDict(sysmod);