Use Py_TPFLAGS_IMMUTABLETYPE to check for class assignments

Author: Erlend E. Aasland

Objects/typeobject.c

@@ -4737,10 +4737,10 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
     */
     if (!(PyType_IsSubtype(newto, &PyModule_Type) &&
           PyType_IsSubtype(oldto, &PyModule_Type)) &&
-        (!(newto->tp_flags & Py_TPFLAGS_HEAPTYPE) ||
-         !(oldto->tp_flags & Py_TPFLAGS_HEAPTYPE))) {
+        (_PyType_HasFeature(newto, Py_TPFLAGS_IMMUTABLETYPE) ||
+         _PyType_HasFeature(oldto, Py_TPFLAGS_IMMUTABLETYPE))) {
         PyErr_Format(PyExc_TypeError,
-                     "__class__ assignment only supported for heap types "
+                     "__class__ assignment only supported for mutable types "
                      "or ModuleType subclasses");
         return -1;
     }