Ported to Python 3

This makes mpDris2 work with Python 3 while retaining backwards compatibility with Python2.

  * The largest change is that `MPDWrapper` does not extend MPDClient directly anymore – wrapping the MPD commands has become almost impossible with the latest python-mpd. Instead of having three different overrides for _execute or _docommand, it just forwards the calls to `self.client` now.