feat(fetchart): add support for webp files

[ShimmerGlass]

Description

Add support for webp images in the fetchart plugin.

To Do

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

[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 70.26%. Comparing base (2dff447) to head (e49398b).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6511   +/-   ##
=======================================
  Coverage   70.26%   70.26%           
=======================================
  Files         147      147           
  Lines       18752    18752           
  Branches     3059     3059           
=======================================
  Hits        13177    13177           
  Misses       4927     4927           
  Partials      648      648           

Files with missing lines	Coverage Δ
beetsplug/fetchart.py	74.24% <100.00%> (ø)

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[snejus]

Looks good, just needs a note in the changelog. Could you double-check whether any docs need updating, also taking into account your PR from yesterday?

[ShimmerGlass]

@snejus looking at the changelog update from yesterday's PR, I think this should be moved to bug fixes. do you agree?
- :doc:`plugins/fetchart`: Error when a configured source does not exist or sources configuration is empty.

[snejus]

@snejus looking at the changelog update from yesterday's PR, I think this should be moved to bug fixes. do you agree? - :doc:`plugins/fetchart`: Error when a configured source does not exist or sources configuration is empty.

Looks good. Did you check fetchart plugin documentation whether any of it needs updating?

[ShimmerGlass]

yup, I made a small change to the fetchart doc

[snejus]

Nice, thanks!