Use GLib.io_add_watch() instead of IOChanell.add_watch()

The add_watch() method has been deprecated in pygobject-3.7.2

Closes rhbz#985990

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>