Drop support for Python 3.9

[browniebroke]

Reaches EOL at the end of the month

https://peps.python.org/pep-0596/#lifespan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7a95be6) to head (ed9a0b6).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #183   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          400       400           
  Branches        54        54           
=========================================
  Hits           400       400           

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