Fix ci

[Chilipp]

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

see psyplot/psyplot#58

Closes #32 (remove if there is no corresponding issue, which should only be the case for minor changes)

• 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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (227d84e) 78.09% compared to head (0dc9bc1) 78.16%.

❗ Current head 0dc9bc1 differs from pull request most recent head cf82b53. Consider uploading reports for the commit cf82b53 to get more accurate results

Files	Patch %	Lines
psyplot_gui/backend.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #33      +/-   ##
===========================================
+ Coverage    78.09%   78.16%   +0.07%     
===========================================
  Files           17       17              
  Lines         5190     5198       +8     
===========================================
+ Hits          4053     4063      +10     
+ Misses        1137     1135       -2     

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