More comprehensive testing for ABC

[ogrisel]

This improves the test that check the support for ABC by:

• using a natural Python 3 syntax to define the ABC class now that Python 2 support has been dropped;
• checks interactions between @abstractmethod and @staticmethod and @classmethod.

Related to the discussion in #367.

[codecov]

Codecov Report

Merging #371 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #371   +/-   ##
=======================================
  Coverage   92.95%   92.95%           
=======================================
  Files           2        2           
  Lines         809      809           
  Branches      164      164           
=======================================
  Hits          752      752           
  Misses         29       29           
  Partials       28       28           

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