Tests

[arkkors]

This merge request adds some tests written for the pytest framework, bringing the coverage to sane ~70% (hopefully).
It also adds all required changes to make unit-testing feasible.

This means:

• pinning protobuf to a version from 2019
• s/Spectrum2/spectrum2/g
• fixing some bugs which make unittesting much harder, that is:
  • add a guard to transWhat.session.Session:logout, otherwise everything crashes and burns on __del__
• adding tox and friends

Note that this MR has not the goal to get everything working (yet), just to get an usable working base.

Progress (Coverage >= 70%):

• deferred
• bot
• buddy
• group
• registersession
• session
• threadutils
• transwhat
• whatsappbackend
• yowsupwrapper