See mopidy#22 (mopidy#22)

It also addresses the comments in mopidy#236

- can't pass in sets, only lists
- provide a useful default n parameter

making playlist private, not just for the obvious reason, but also
because our API client is not authorized for the playlist-modify-public
scope (only playlist-modify-private).

not thoroughly tested yet

Uses Myers' diff, extended to detect moved ranges. With the exception of
non-consecutive deletes, this generates a near-optimal edit script.

To use the preexisting mopidy_album_mock the web translator had to be
extended to parse release dates (only the year is supported, as that's
what the test data requires).

https://developer.spotify.com/community/news/2018/06/12/changes-to-playlist-uris/
https://developer.spotify.com/community/news/2019/01/15/update-changes-to-playlist-uris/

op.__repr__ is useful for debugging only.

this won't ever fire, since getattr() will raise on error.

Rest will be fixed by black.

... and its accompanying tests.

note that these functions are not part of the SpotifyOAuthClient class,
so the web_client_mock will use them, instead of a mocked version of
them.

Comment from @kingosticks:
> Tracks might be currently unplayable if unavailable in the geo region
> where the user is logged in (but may be playable from elsewhere and/or
> in the future). These unplayable tracks will clutter the
> queue/playlist listing and might be confusing to users when they try
> to play them. But I don't see another way to avoid losing them when we
> use save().

Since the Spotify Web API does not support adding tracks, we can't make
use of the 'replace' strategy. And removing local tracks by URL+position
doesn't work either (according to some vague docs), so we'd have to
rework not just the DELETE API call, but also delta counting and
implement fetching of additional playlist metadata.

further reading:
https://community.spotify.com/t5/Spotify-for-Developers/API-Get-a-Playlist-s-Items-Incorrect-tracks/td-p/4990167
https://developer.spotify.com/documentation/general/guides/local-files-spotify-playlists/

…o bug22

…o bug22