Add spotify as a fetchart source

[arsaboo]

Added spotify as a fetchart source.

Let me know if this approach looks fine.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[wisp3rwind]

Looks reasonable at a first glance 🎉 (Without checking anything Spotify-specific.) One small thing: We list beautifulsoup as an optional dependency, so we need to import in conditionally. See for example the lyrics plugin and the dependency specification in setup.py.

[arsaboo]

Thanks for the review @wisp3rwind. I have updated the code to make beautifulsoup optional and added a changelog.

[wisp3rwind]

Looking good! Would you mind rebasing on (or merging) master in order to get #4772 for a proper CI run?

[arsaboo]

Thanks @wisp3rwind for that fix...all green now ✅

[wisp3rwind]

Great! I didn't check whether the Spotify "API" usage is correct, but since this is well-contained and otherwise the code looks good, I'm just going ahead and merge.