Fix UBSan error in _testcapi: correct create_attr_from_spec signature

Author: ashm-dev

Modules/_testcapi/module.c

@@ -180,7 +180,8 @@ module_from_slots_exec(PyObject *self, PyObject *spec)
 static PyObject *
 create_attr_from_spec(PyObject *spec, PyObject *def)
 {
-    assert(!def);
+    PyModuleDef *module_def = (PyModuleDef *)def;
+    assert(!module_def);
     return PyObject_GetAttrString(spec, "_gimme_this");
 }