⬆️ Drop Python 3.9

[ubmarco]

Required as a preparation for #1467.
Python 3.9 reaches its end of life on 2025-10.

The supported Sphinx versions are not touched.

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.60%. Comparing base (4e10030) to head (9e71701).
⚠️ Report is 176 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needbar.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1468      +/-   ##
==========================================
+ Coverage   86.87%   88.60%   +1.72%     
==========================================
  Files          56       61       +5     
  Lines        6532     7259     +727     
==========================================
+ Hits         5675     6432     +757     
+ Misses        857      827      -30     

Flag	Coverage Δ
pytests	88.60% <97.50%> (+1.72%)	⬆️

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[danwos]

Thanks for the update. 👍

[chrisjsewell]

all good cheers