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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