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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