Test against & support Python 3.9

[jwodder]

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[codecov]

Codecov Report

Merging #297 (12630c4) into master (c7aac72) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #297   +/-   ##
=======================================
  Coverage   82.13%   82.13%           
=======================================
  Files          55       55           
  Lines        4640     4640           
=======================================
  Hits         3811     3811           
  Misses        829      829           

Flag	Coverage Δ
unittests	82.13% <ø> (ø)

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