Drop Python 3.6 and 3.7, update test workflows

[pllim]

Description

This pull request is to update testing workflows and fix #324

[github-actions]

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[codecov]

Codecov Report

Merging #330 (e43c2a6) into master (e51d760) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files          14       14           
  Lines        2005     2005           
=======================================
  Hits         1891     1891           
  Misses        114      114           

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