gh-117657: Enable test_opcache under TSAN (GH-129831)

Fix a few thread-safety bugs to enable test_opcache when run with TSAN:

 * Use relaxed atomics when clearing `ht->_spec_cache.getitem`
   (gh-115999)
 * Add temporary suppression for type slot modifications (gh-127266)
 * Use atomic load when reading `*dictptr`

Objects/object.c

@@ -1612,7 +1612,7 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
     else {
         PyObject **dictptr = _PyObject_ComputedDictPointer(obj);
         if (dictptr != NULL) {
-            dict = *dictptr;
+            dict = FT_ATOMIC_LOAD_PTR_ACQUIRE(*dictptr);
         }
         else {
             dict = NULL;