Drop 3.9 support + add 3.14 CI

- Update pyproject.toml etc. - Add to CI runs - Drop 3.9 as flake8 itself did Test: - Run tox in 3.14 - `python3 -m venv /tmp/tf --upgrade-deps` - `/tmp/tf/bin/pip install tox` - `/tmp/tf/bin/tox -e py314`