Add priorities possibility for messages in pubsub

v0.4 : 20 dec. 2020 by Thierry Maillard (Thierry46)
- just warn when queue overflows when publishing in a channel
- implement PubSubPriority to register messages with priorities
- only support Python >= 3.6 but not Python 2
- improve quality metrics
- remove listen method in main PubSub communicator class
use ChanelQueue*.listen() instead.
- replace functool.partial use by ChanelQueue* class

Improve quality and add test cases

• v0.3:
  • Improved explanation of parameter max_queue_in_a_channel with comments.
  • Addition of a new test case with listeners and senders working in threads.
  • Placement of test cases in a separate directory.
  • Addition of new test cases to check possible usage problems.
  • Restoration of original behaviour when a channel overflows.
  • Correction of spelling mistakes.

V0.2 : Add class container and doc

• 0.2:
  • Embbed original version written by Zhen Wang in a class to use it as an object in different file sources
  • Add more comments
  • Adapt unit tests written by Zhen Wang and add new ones
  • Use pytest to run unit test