Accept lyrics source as a string (#6149)

## Description

Fixes #5962.

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

beetsplug/lyrics.py

@@ -958,7 +958,7 @@ class LyricsPlugin(RequestHandler, plugins.BeetsPlugin):
 
     @cached_property
     def backends(self) -> list[Backend]:
-        user_sources = self.config["sources"].get()
+        user_sources = self.config["sources"].as_str_seq()
 
         chosen = sanitize_choices(user_sources, self.BACKEND_BY_NAME)
         if "google" in chosen and not self.config["google_API_key"].get():

docs/changelog.rst

@@ -29,6 +29,8 @@ Bug fixes:
   audio-features endpoint, the plugin logs a warning once and skips audio
   features for all remaining tracks in the session, avoiding unnecessary API
   calls and rate limit exhaustion.
+- :doc:`plugins/lyrics`: Accepts strings for lyrics sources (previously only
+  accepted a list of strings). :bug:`5962`
 
 For plugin developers: