python3Packages.beets: mark bpsync plugin deprecated

The plugin depends on the retired Beatport API and is no longer functional.

Assisted-by: Codex (GPT-5)
(cherry picked from commit bfbb7882ca)

pkgs/development/python-modules/beets/default.nix

@@ -279,6 +279,8 @@ buildPythonPackage (finalAttrs: {
         bpd = { };
         bpm.testPaths = [ ];
         bpsync = {
+          # plugin retired: https://github.com/beetbox/beets/issues/3862.
+          deprecated = true;
           testPaths = [ ];
         };
         bucket = { };