Add Python v3.14 support and drop Python v3.9

Python 3.14 pre-releases

• Set up v3.14 test job
  • With allow_failure: true, to allow tests to fail

Python 3.14 stable release

• Handle deprecation warning and breaking features
• Remove allow_failure: true, now on the tests should pass

Drop Python 3.9 support

• Delete test job for v3.9 from .gitlab-ci.yml
• pyproject.toml: Bump minimum version requires-python
• .pre-commit-config.yaml:pyupgrade/tools.ruff.target-version
• Update minimal version in documentation: README.md and docs/index.rst

Post

• Open follow-up v3.10/3.15 ticket: Add Python v3.15 support and drop Python v3.10 (#216)

References

• Expected to be release on October 7th 2025.
• Introduce 3.14 and drop 3.9 Python image genera... (docker/python3/poetry#12 - closed)