remove python 3.9 in tests and docs

[ksbeattie]

Fixes/Addresses:

Python 3.9 has been EOL'd. This removes python 3.9 from tests and docs.

Summary/Motivation:

This is needed for the black upgrade (PR #1283).

Changes proposed in this PR:

Legal Acknowledgement

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.87%. Comparing base (968f800) to head (ef62250).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1284      +/-   ##
==========================================
+ Coverage   39.86%   39.87%   +0.01%     
==========================================
  Files         165      165              
  Lines       37747    37747              
  Branches     5865     5865              
==========================================
+ Hits        15048    15052       +4     
+ Misses      21599    21597       -2     
+ Partials     1100     1098       -2     

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