Fix build with SWIG 4.3.0

[sunpoet]

Replace SWIG_Python_AppendOutput with SWIG_AppendOutput.

from CHANGES:

            New declaration of SWIG_Python_AppendOutput is now:

              SWIG_Python_AppendOutput(PyObject* result, PyObject* obj, int is_void);

            The 3rd parameter is new and the new $isvoid special variable
            should be passed to it, indicating whether or not the wrapped
            function returns void. If SWIG_Python_AppendOutput is currently being
            used and a completely backwards compatible (but technically incorrect)
            solution is required, then pass 1 for the is_void parameter.

            Also consider replacing with:

              SWIG_AppendOutput(PyObject* result, PyObject* obj);

            which calls SWIG_Python_AppendOutput with same parameters but adding $isvoid
            for final parameter.