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 (4b69544).

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              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rettinghaus]

@bertsky I want a review for christmas. 😉