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- For the replacement with NULL, Py_CLEAR() should be used instead.
- We should use a macro (Py_REF_ASSIGN?) for the replacement case.
- Careful, in Modules/_json.c the code is wrong because tmp is already used::
 PyObject *tmp = PyUnicode_AsEncodedString(...);
 {
 PyObject *tmp = s->encoding;
 s->encoding = tmp;
 Py_DECREF(tmp);
 }