MNT add py39 support

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: MNT support Python 3.9 #195
• A description of the changes: add python 3.9 support and tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fac6872) to head (90202bb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #196   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          687       689    +2     
=========================================
+ Hits           687       689    +2     

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