bt2: use SWIG_AppendOutput (fix for SWIG 4.3.0)

bt2: use SWIG_AppendOutput (fix for SWIG 4.3.0)

From the SWIG release notes:

  [Python] #2907 Fix returning null from functions with output
  parameters.  Ensures OUTPUT and INOUT typemaps are handled
  consistently wrt return type.

  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.

Change-Id: I0944116fa40e024785d48e0b172ab30f359f505c
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13345
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>