Keep `runtime.txt` files updated.

Currently supports `Python` runtime updates, commonly used by platforms such as

- [Heroku](https://devcenter.heroku.com/articles/python-runtimes)
- [CloudFoundry](https://docs.cloudfoundry.org/buildpacks/python/index.html)
- [Koyeb](https://www.koyeb.com/docs/build-and-deploy/build-from-git/python)
- and more
