Prepare release 2026.05.09

- Bump __version__ to 2026.05.09 and regenerate CHANGELOG.md.
- Add Python 3.12 / 3.13 / 3.14 classifiers in setup.py to match
  the CI test matrix. Last release with 3.8 / 3.9 classifiers.
- Replace the stale .travis.yml link in CONTRIBUTING.md with the
  actual release workflow at .github/workflows/pypi-release.yml.

Author: AndreMiras