Add CoreListner to backend

mopidy_smartplaylists/backend.py

@@ -1,14 +1,14 @@
 import pykka
 
 import logging
-from mopidy import backend
+from mopidy.core import CoreListener
 from .playlists import SmartPlaylistsProvider
 
 
 logger = logging.getLogger(__name__)
 
 
-class SmartPlaylistsBackend(pykka.ThreadingActor, backend.Backend):
+class SmartPlaylistsBackend(pykka.ThreadingActor, CoreListener):
     def __init__(self, config, audio):
         super(SmartPlaylistsBackend, self).__init__()
         self.config = config["smartplaylists"]