Update tests / CI version matrix to cover 3.14

[lukehinds]

Is your feature request related to a problem? Please describe.

3.14 has shipped, so we need to update the version matrix for unit tests, and CI

• Add Python 3.14 Trove classifier in setup.cfg.
• Add Python 3.14 to CI matrix in .github/workflows/pythonpackage.yml.
• Add py314 env to tox.ini and include in envlist.
• Upgrade pylint and test deps to 3.14-compatible versions.
• Verify runtime deps (PyYAML, stevedore, rich) on 3.14.
• Run tests on 3.14 with PYTHONWARNINGS=error and fix issues.
• Update docs/FAQ to mention Python 3.14 support.
• Update Docker base image to python:3.14-alpine (if shipping image).
• Update CONTRIBUTING.md to include tox run -e py314.
• Add release notes announcing official Python 3.14 support.
• Might want to update pre-commit targets to --py39-plus/--target-version=py39.

Related issues:

#1311