Mopidy-beets

- Add Python 3.9 to the test matrix. (PR: 32)

- No changes since 4.0.0rc2.

- Fix casing of PyPI distribution name.

- Require Mopidy >= 3.0.0a3.

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

- Update project setup.

- Change default port from `8888` to `8337` to match Beets' defaults.

- Fix handling of non-ASCII characters in album titles and artist names.

- Fix handling of empty titles and names.

- Reduce resource consumption of string matching API requests.

- Support browsing albums by artist, genre and year.

- Improved search (more categories, more precise).

- Align with Mopidy's current extension guidelines.