Mopidy-pandora

- Move changelog to GitHub Releases.
- Test new automatic release to PyPI.

- Add station art. (Fixes: 74, PR: 75)
- Fix crash when `quickmix_stations` is undefined. (PR: 77)
- Add Python 3.9 to the test matrix.

- Now requires Python 3.7 or newer. Python 2.7 is no longer supported.
- Now requires Mopidy 3.0 or newer.
- Switch to Black code formatting.
- Update docs to refer to the new Pandora 'Premium' subscription model instead of the old 'Plus'.

- Pin pydora dependency to pydora>=1.13,<2. (Fixes: 70).

- Update package dependencies used during development to the latest versions.
- Migrate all tests to pytest. Resolve pytest 4.0 deprecation errors.
- Switch to Xenial Travis build environment.

Fixes

- Use the updated Station API introduced in pydora 1.12.0. (Fixes: 65).
- Implement `LRUCache`'s `__missing__`. (Fixes: 66).

- Update documentation to refer to new 'Pandora Plus' subscription model instead of the old 'Pandora One'.
- Update troubleshooting guide with more workarounds for cross-signed certificates using OpenSSL < 1.0.2.
- Allow station URI's to be added to playlists and the Mopidy tracklist. (Addresses: 58).