Drop support for Python 3.9

[jacobtylerwalls]

Type of Changes

	Type
✓	🔨 Refactoring

Description

Pylint already dropped support for Python 3.9, so we can do it here, too. Python 3.9 EOL is October 2025. Unlikely to have a new pylint feature release before then.

[codecov]

Codecov Report

❌ Patch coverage is 98.70130% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.26%. Comparing base (c94e0b7) to head (75ad3a3).
⚠️ Report is 105 commits behind head on main.

Files with missing lines	Patch %	Lines
astroid/interpreter/_import/spec.py	91.66%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (98.70%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2778      +/-   ##
==========================================
- Coverage   93.31%   93.26%   -0.06%     
==========================================
  Files          92       91       -1     
  Lines       11051    11010      -41     
==========================================
- Hits        10312    10268      -44     
- Misses        739      742       +3     

Flag	Coverage Δ
linux	93.12% <98.70%> (-0.07%)	⬇️
pypy	93.26% <98.70%> (-0.06%)	⬇️
windows	93.24% <98.70%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
astroid/__init__.py	100.00% <100.00%> (ø)
astroid/brain/brain_builtin_inference.py	92.03% <100.00%> (ø)
astroid/brain/brain_dataclasses.py	93.31% <100.00%> (-0.05%)	⬇️
astroid/brain/brain_ssl.py	100.00% <100.00%> (ø)
astroid/brain/brain_subprocess.py	100.00% <100.00%> (ø)
astroid/const.py	100.00% <ø> (ø)
astroid/constraint.py	100.00% <100.00%> (ø)
astroid/context.py	97.43% <100.00%> (ø)
astroid/decorators.py	79.12% <100.00%> (+1.46%)	⬆️
astroid/modutils.py	89.61% <100.00%> (-0.08%)	⬇️
... and 8 more

... and 4 files with indirect coverage changes

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[jacobtylerwalls]

I'll add the last typing fixes.

[Pierre-Sassoulas]

Reading this is like meditation, I feel refreshed.