Fix issue using basestring

Checking against basestring was necessary in python two since unicode and string were separate classes. In python3 basestring was removed since different string classes have been removed.

mycroft/skills/audioservice.py

@@ -78,7 +78,7 @@ class AudioService(object):
                            audio service.
         """
         tracks = tracks or []
-        if isinstance(tracks, basestring):
+        if isinstance(tracks, str):
             tracks = [tracks]
         elif not isinstance(tracks, list):
             raise ValueError