Mopidy-nad

- Add Python 3.9 to the test matrix. (PR: 4)

- Depend on final release of Mopidy 3.0.0.

- Require Mopidy >= 3.0.0a5. (PR: 3)

- Require Python >= 3.7. (PR: 3)

- Update project setup. (PR: 3)

- Require Mopidy >= 0.19 and the new Mopidy mixer API.

- The configuration format has changed, due to the move from GStreamer 0.10's
mixer API to Mopidy's new mixer API.

- Require Mopidy >= 0.18.

- Update to match the `cookiecutter-mopidy-ext` project template.