Update unittests, etc

[jGaboardi]

This PR:

• adds Python 3.9 to testing schema
• update and standardizes unittests.yml and release_and_publish.yml
  • uses micromamba action for much speedier env resolves

[jGaboardi]

@sjsrey we may want to take this opportunity to drop support for Python 3.6. Thoughts?

[sjsrey]

@jGaboardi should we do another post release to fix docs?

[jGaboardi]

Yes, now the manual trigger is available let's try to bump another post release and trigger a doc build. After that we should be able to change the page source to the gh-pages/root branch.

[sjsrey]

I will do the bump and post.