Drop support of Python 3.9

[dbaty]

Description

Python 3.9 has reached its end of life 6 months ago (2025-10-31).

See https://devguide.python.org/versions/

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected (this may include manually testing them in your local environment, or in other available environments). Cross this out if not relevant (for example, if you're making a documentation change).
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[j-bennet]

This now has conflicts. Feel free to merge after you rebase!

[dbaty]

This now has conflicts. Feel free to merge after you rebase!

@j-bennet : I have rebased the branch.

[j-bennet]

Integration tests are flaky lately. :(

[j-bennet]

Thanks @dbaty , nice work!