fix ci

[Chilipp]

this PR aims to fix the CI builds and make some compatibility fixes

see psyplot/psyplot#58

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0479a55) 78.55% compared to head (e8c1bbb) 78.26%.

❗ Current head e8c1bbb differs from pull request most recent head 2864852. Consider uploading reports for the commit 2864852 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #46      +/-   ##
===========================================
- Coverage    78.55%   78.26%   -0.30%     
===========================================
  Files            6        6              
  Lines         1357     1357              
===========================================
- Hits          1066     1062       -4     
- Misses         291      295       +4     

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