Mopidy-scrobbler

- Add Python 3.9 to the test matrix. (PR: 35)

- Depend on final release of Mopidy 3.0.0.

- Require Mopidy >= 3.0.0a4. No major changes required.

- Require Python >= 3.7. No major changes required.

- Require pylast >= 2.2, as that's the first version to support Python 3.7.

- Update project setup.

- Require pylast < 3, as that version removed support for Python 2.7. (Fixes: 30)

- Require pylast >= 1.6.0, which is the version packed in Debian stable.

- Fix compatability with pylast >= 2, which has removed the `ScrobblingError`
exception type.

- Updated to work with `None` as the default value of `track_no` in
Mopidy's `Track` model. This was changed in Mopidy 0.19.5. (Fixes: 7)