text: Title: publish_subscribe.py - Method missing?
  Body: The example does not make use of the method Provider.unsubscribe().
  
  I expected that there would be a "wrapping" method like Subscriber.unsubscribe() for removing itself from the Provider (compare: Subscriber.subscribe() for appending itself to the Provider)?
  
  [See draft for publish_subscribe.py tests, line 24](https://github.com/fkromer/python-patterns/blob/test_publish_subscribe/test_publish_subscribe.py)
  