old_contents: from __future__ import absolute_import, division, print_function, unicode_literals
  
  import abc
  
  
  class MessageReceiver(object):
  
      __metaclass__ = abc.ABCMeta
  
      @abc.abstractmethod
      def process_message(self, message):
          raise NotImplementedError
  
      def setup_hook(self, conf):
          pass
  
new_contents: from __future__ import absolute_import, division, print_function, unicode_literals
  from six import with_metaclass
  
  import abc
  
  
  class MessageReceiver(with_metaclass(abc.ABCMeta, object)):
  
      @abc.abstractmethod
      def process_message(self, message):
          raise NotImplementedError
  
      def setup_hook(self, conf):
          pass
  
subject: enforce MessageReceiver metaclass for python2 and 3 using six with_metaclass