Add the ability to handle error 429 (too many requests) in Spotify plugin

[arsaboo]

Spotify has a rate-limiting feature. If there are too many requests to the Spotify App it will prevent you from sending further requests. Fortunately, the response includes the retry-after value that we should be using to back off.

I am just creating this issue so that we can keep a track of it based on @wisp3rwind comment here.

Here are some reference links to implement this feature:
skuethe/MMM-Spotify#172
felix-hilden/tekore#163

[sampsyo]

Good idea! An ideal implementation of rate limiting would work by measuring the time since the last call to the function—if the calls are naturally spaced out by some other work to be done, then there should be no need to sleep at all.

Something along the lines of the rate-limiting in the MusicBrainz library we use:
https://github.com/alastair/python-musicbrainzngs/blob/1638c6271e0beb9560243c2123d354461ec9f842/musicbrainzngs/musicbrainz.py#L376

…although I wrote that code a very long time ago, and maybe there are better Python stdlib facilities for this kind of pattern now?