Update pythonpackage.yml

[jgrewe]

remove 3.6, 3.7 python versions since they reached end of life from matrix and add newer versions

[coveralls]

coverage: 79.78% (-0.006%) from 79.786%
when pulling fc65fe3 on jgrewe-actions
into 481847b on master.

[codecov-commenter]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (481847b) 81.36% compared to head (fc65fe3) 81.36%.

Files	Patch %	Lines
nixio/cmd/explore.py	0.00%	34 Missing ⚠️
nixio/test/xcompat/compile.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
- Coverage   81.36%   81.36%   -0.01%     
==========================================
  Files          81       81              
  Lines       10825    10827       +2     
==========================================
+ Hits         8808     8809       +1     
- Misses       2017     2018       +1     

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