chore: fix playlist list function within util

...that is actually unused but was copied here from the beetstream project and may be synchronized between the projects subsequentially

beetsplug/webm3u/playlist.py

@@ -31,7 +31,7 @@ def _load_playlists(self):
     def playlists(self):
         self._refresh()
         playlists = self._playlists
-        ids = [k for k, v in playlists if v]
+        ids = [k for k, v in playlists.items() if v]
         ids.sort()
         return [playlists[id] for id in ids]