mopidy/ci-python removed from Docker Hub

[jodal]

Background: https://blog.alexellis.io/docker-is-deleting-open-source-images/

The mopidy/ci-python repo on Docker Hub has not been in use since Mopidy's extensions used Circle CI for testing years ago, thus it has simply been removed as a response to Docker's push towards paid accounts.

If you are maintaining a Mopidy extension that uses Circle CI and the mopidy/ci-python image, please consider migrating to GitHub Actions and the Docker image hosted at GitHub. Any extension in the Mopidy GitHub org or the Mopidy extension cookiecutter can be used a starting point.