PyObject *exception, *value, *tb;
        PyErr_Fetch(&exception, &value, &tb);
        /* PyObject_IsSubclass() can recurse and therefore is
           not safe (see test_bad_getattr in test.pickletester). */
        res = PyType_IsSubtype((PyTypeObject *)err, (PyTypeObject *)exc);
        /* This function must not fail, so print the error here */
        if (res == -1) {
            PyErr_WriteUnraisable(err);
            res = 0;
        }
        PyErr_Restore(exception, value, tb);

According to the comment, there was previously a call to PyObject_IsSubclass() involved which could fail, but since it was replaced with a call to PyType_IsSubtype(), it can no longer fail.
See pull request.