Xpra: Ticket #2365: macos python2 sound query failures

This triggers during the unit tests with python2.

We want "gtk3" (actually the gi bindings) to use gstreamer, but the code first loads the default xpra config which in turn loads the gtkosx_application module and uses the non-gi version... then we can't load gi:

xpra initialization error:
 cannot set gtk3=True, already set to False
  File "/Users/osx/gtk/inst/lib/python2.7/site-packages/xpra/scripts/main.py", line 106, in main
    return run_mode(script_file, err, options, args, mode, defaults)
  File "/Users/osx/gtk/inst/lib/python2.7/site-packages/xpra/scripts/main.py", line 481, in run_mode
    return run_sound(mode, error_cb, options, args)
  File "/Users/osx/gtk/inst/lib/python2.7/site-packages/xpra/sound/wrapper.py", line 147, in run_sound
    want_gtk3(True)
  File "/Users/osx/gtk/inst/lib/python2.7/site-packages/xpra/gtk_common/gobject_compat.py", line 43, in want_gtk3
    assert (_is_gtk3 is None or _is_gtk3==v), "cannot set gtk3=%s, already set to %s" % (v, _is_gtk3)