Streamline & upgrade CI

[jGaboardi]

This PR addresses #124 and #134.

• fixes minor change in conf.py
• swaps out conda for micromamba in CI (much quicker env resolves)
• defaults to testing with stable versions of deps on all OSs & Python versions
• tests bleeding edge deps only on Ubuntu & Python 3.9

[codecov-commenter]

Codecov Report

Merging #135 (c2d3a04) into main (414386e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   85.94%   85.94%           
=======================================
  Files          18       18           
  Lines        1217     1217           
=======================================
  Hits         1046     1046           
  Misses        171      171           

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[jGaboardi]

@slumnitz Let's merge this unless you have any objections?