chore: drop support for Python 3.9 (EOL)

[bhimrazy]

What does this PR do?

Deprecates and removes support for Python 3.9, which reached end-of-life on October 25th, 2025.
Reference: Python Developer’s Guide – Supported Versions

follow-up to #747

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80%. Comparing base (b070032) to head (4a8bbd8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #751   +/-   ##
===================================
  Coverage    80%    80%           
===================================
  Files        52     52           
  Lines      7343   7343           
===================================
+ Hits       5885   5891    +6     
+ Misses     1458   1452    -6     

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