CI: fix the test workflow

[maximlt]

For an unknown reason it is no longer solving on Python 3.6 on one platform. Attempting to fix that.

EDIT: Really struggled to get the environment to solve on Python 3.6, and I still don't know why. mamba helped.

[codecov-commenter]

Codecov Report

Merging #87 (7f0ac69) into master (3cdbb67) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   85.78%   85.78%           
=======================================
  Files           8        8           
  Lines        1914     1914           
=======================================
  Hits         1642     1642           
  Misses        272      272           

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[jbednar]

Thanks so much!