About _json_encoder_init.patch, an alternative patch is to write
arguments references (addresses) in local variables, and only copy them
on success. Something like:
  PyObject *arg;
  if (!PyTuple_ParseArgs(..., &arg)) return NULL;
  Py_INCREF(arg);
  self->arg = arg;

I prefered to write a shorter patch.

Tell me if you prefer the alternative fix, and if you would like unit tests.