remove Python 3.7 from test workflow

[jkisse]

Python 3.7 tests are currently failing and there is not really a need to fix them. This PR adapts the GitHub Actions script so that Python 3.7 will no longer be tested.
closes #499

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (9527ded) 85.06% compared to head (bdd89f9) 85.06%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #524   +/-   ##
========================================
  Coverage    85.06%   85.06%           
========================================
  Files           90       90           
  Lines         6080     6080           
========================================
  Hits          5172     5172           
  Misses         908      908           

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[dlohmeier]

Since pandapower now introduces python 3.8 specific implementations (like string formatting notation), this makes sense and should encourage to adapt such kind of notations, as they are easier to implement and read.