update doc build to python 3.8 only & add python 3.9 to build

[lwasser]

This closes #481
The issue here is that the pydata sphinx theme doesn't support python 3.6 which is end of life.
We probably want to remove support for 3.6 at this point and add support for 3.9 but we may not have time to do this as we have to wrap up maintenance of this package for the time being.

For now we are simply removing the build of the docs to allow for other PR's to successfully build. But in the longer term someone will need to go in here and decide to remove python 3.6 from support.

[codecov]

Codecov Report

Merging #485 (57bce21) into main (941592f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #485   +/-   ##
=======================================
  Coverage   60.47%   60.47%           
=======================================
  Files          20       20           
  Lines        1169     1169           
=======================================
  Hits          707      707           
  Misses        462      462           

Flag	Coverage Δ
unittests	60.47% <ø> (ø)

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[lwasser]

@kcranston @nkorinek this is the fix - i just made the build ONLY build on python 3.8. should we consider adding a build for 3.9 ? just lemme know what you think but this could be merged as is too. it's a simply fix .