Turn on python 3.9 QA and fix typing

[gmertes]

• Turn on python 3.9 in the QA version matrix
• type | None is unsupported in python 3.9, replace it with Optional[type] or Union[type, type, None]

Other instances of | do work thanks to __future__.annotations , so I left those alone.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (90728d5) to head (b10dce8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #104   +/-   ##
========================================
  Coverage    98.03%   98.03%           
========================================
  Files            3        3           
  Lines           51       51           
========================================
  Hits            50       50           
  Misses           1        1           

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