old_contents: class RepresentationMixin(object):
      @property
      def react_representation(self):
          raise NotImplementedError(
              'Missing property react_representation in class'
          )
  
new_contents: class RepresentationMixin(object):
      def to_react_representation(self, context=None):
          raise NotImplementedError(
              'Missing property to_react_representation in class'
          )
  
subject: Switch NotImplementedError to to_react_represetnation in mixin