Drop Python 3.9

[tony]

Changes

Drop Python 3.9

EOL of 2025-10-31.

See also:

• https://devguide.python.org/versions/#:~:text=Release%20manager-,3.9,-PEP%20596
• https://peps.python.org/pep-0596/

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.24%. Comparing base (373296a) to head (1a2484d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   76.24%   76.24%           
=======================================
  Files           5        5           
  Lines         181      181           
=======================================
  Hits          138      138           
  Misses         43       43           

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