Accept lyrics source as a string

[DogeTheBeast]

Description

Fixes #5962 .

The fix was shared in the issue. Now it uses as_str_seq similarly to other plugins.

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.)

[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location> `docs/changelog.rst:33-34` </location>
<code_context>
+- :doc:`plugins/lyrics`: Accepts strings for lyrics sources (used to only accept
+ list of strings).  :bug:`5962`
</code_context>

<issue_to_address>
**suggestion (typo):** Minor grammatical issue: missing article in 'used to only accept list of strings'.

Change to 'previously only accepted a list of strings' for correct grammar.

```suggestion
- :doc:`plugins/lyrics`: Accepts strings for lyrics sources (previously only accepted a list of strings).  :bug:`5962`
```
</issue_to_address>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.55%. Comparing base (61a4c73) to head (26a8e16).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6149      +/-   ##
==========================================
+ Coverage   67.34%   67.55%   +0.21%     
==========================================
  Files         136      136              
  Lines       18492    18492              
  Branches     3134     3134              
==========================================
+ Hits        12453    12493      +40     
+ Misses       5370     5329      -41     
- Partials      669      670       +1     

Files with missing lines	Coverage Δ
beetsplug/lyrics.py	86.61% <100.00%> (+8.11%)	⬆️

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[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[DogeTheBeast]

This is my first PR. Please feel free to give feedback or things I should change.

[semohr]

Thank you for the PR!

We're thrilled to have you contributing, welcome to the community!