session: Move sp_session_callbacks into _SessionCallbacks
session: Move sp_session_callbacks into _SessionCallbacks

felix1m/pyspotify,jodal/pyspotify,mopidy/pyspotify,jodal/pyspotify,mopidy/pyspotify,felix1m/pyspotify,felix1m/pyspotify,kotamat/pyspotify,kotamat/pyspotify,jodal/pyspotify,kotamat/pyspotify