Fix python 3.9 dev dependencies

[leszekhanusz]

Fixing two problems on Python 3.9:

• The | syntax for typing Union is not yet supported on Python 3.9
• sphinx-argparse==0.5.2 dev dependency does not exist for Python 3.9

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (033a40c).
⚠️ Report is 40 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #568    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3235   +327     
==========================================
+ Hits          2908      3235   +327     

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