Drop Python 3.8 on CircleCI

[rettinghaus]

closes #79

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.80%. Comparing base (f899803) to head (8a84172).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   68.80%   68.80%           
=======================================
  Files           9        9           
  Lines        1276     1276           
  Branches      266      242   -24     
=======================================
  Hits          878      878           
  Misses        309      309           
  Partials       89       89           

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[rettinghaus]

superseded by #82