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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.