fetchart: Fix downloading pre-sized thumbnails from CAA

[tibequadorian]

Description

Fixes issue in which thumbnails were rescaled locally instead of downloading pre-sized ones from CAA.

From the docs:

The CAA provides pre-sized thumbnails of width 250, 500, and 1200 pixels. If you set the maxwidth option to one of these values, the corresponding image will be downloaded, saving beets the need to scale down the image. It can also speed up the downloading process, as some cover arts can sometimes be very large.

I noticed that the fetchart plugin didn't download the pre-sized thumbnails from CAA for some of my releases because the API didn't provide them under the keys "500" and "250" but instead "large" and "small". These are legacy keys but for some releases they're the only ones available so I wrote this fix to use them as a fallback.

Example

curl -sL "https://coverartarchive.org/release/d51434bc-fcb6-4fc0-864f-e1ab13a51de2" | jq

{
  "images": [
    {
      "types": [
        "Front"
      ],
      "front": true,
      "back": false,
      "edit": 30140717,
      "image": "http://coverartarchive.org/release/d51434bc-fcb6-4fc0-864f-e1ab13a51de2/8891190287.png",
      "comment": "",
      "approved": true,
      "id": "8891190287",
      "thumbnails": {
        "large": "http://coverartarchive.org/release/d51434bc-fcb6-4fc0-864f-e1ab13a51de2/8891190287-500.jpg",
        "small": "http://coverartarchive.org/release/d51434bc-fcb6-4fc0-864f-e1ab13a51de2/8891190287-250.jpg"
      }
    }
  ],
  "release": "http://musicbrainz.org/release/d51434bc-fcb6-4fc0-864f-e1ab13a51de2"
}

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.84%. Comparing base (39ff8cc) to head (388689d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6870   +/-   ##
=======================================
  Coverage   76.84%   76.84%           
=======================================
  Files         163      163           
  Lines       21626    21627    +1     
  Branches     3343     3343           
=======================================
+ Hits        16619    16620    +1     
  Misses       4191     4191           
  Partials      816      816           

Files with missing lines	Coverage Δ
beetsplug/fetchart.py	74.01% <100.00%> (+0.03%)	⬆️

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[snejus]

Would you mind adding a test for this, where you show that both 200/500 and small/large keys work?

[snejus]

@tibequadorian are you planning to come back to this one?