Modernize test infrastructure

[rettinghaus]

This PR replaces os and distutils packages with pathlib making it work with current and furture Python versions.
This also is the more robust file handling solution across platforms.

Closes #79.

[rettinghaus]

This now uses pytest-order instead of the outdaated pytest-ordering.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.73%. Comparing base (f899803) to head (4b3c038).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   68.80%   68.73%   -0.08%     
==========================================
  Files           9        9              
  Lines        1276     1273       -3     
  Branches      266      241      -25     
==========================================
- Hits          878      875       -3     
  Misses        309      309              
  Partials       89       89              

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[rettinghaus]

@wrznr @bertsky could this get reviewed?

[bertsky]

@rettinghaus sorry, that did slip past me – will review soon – meanwhile, thanks a lot, looks substantial!