Version 2.1.0: Use entry_points for plugins

Bug fixes:

• Fix job --pid under python3 to display output correctly (instead of byte-array
  repr string)
• Stop printing a backtrace when handling KeyboardInterrupt in job -w.

Changes:

• Use the entry point 'wwade.jobrunner' for plugin loading. Deprecate the
  'jobrunner.plugin' module, to be removed in 3.0.0.
• CI updates