old_contents: 
  from vumi.blinkenlights.heartbeat.publisher import (HeartBeatMessage,
                                                      HeartBeatPublisher)
  
  __all__ = ["HeartBeatMessage", "HeartBeatPublisher"]
  
new_contents: """Vumi worker heartbeating."""
  
  from vumi.blinkenlights.heartbeat.publisher import (HeartBeatMessage,
                                                      HeartBeatPublisher)
  
  __all__ = ["HeartBeatMessage", "HeartBeatPublisher"]
  
subject: Add module docstring from vumi.blinkenlights.heartbeat.