old_contents: from mopidy.mixers import BaseMixer
  
  class DummyMixer(BaseMixer):
      """Mixer which just stores and reports the choosen volume."""
  
      def __init__(self):
          self._volume = None
  
      def _get_volume(self):
          return self._volume
  
      def _set_volume(self, volume):
          self._volume = volume
  
new_contents: from mopidy.mixers import BaseMixer
  
  class DummyMixer(BaseMixer):
      """Mixer which just stores and reports the chosen volume."""
  
      def __init__(self):
          self._volume = None
  
      def _get_volume(self):
          return self._volume
  
      def _set_volume(self, volume):
          self._volume = volume
  
subject: Fix typo