python3Packages.beets: init from pkgs.beets (#437915)

pkgs/top-level/python-packages.nix

···
 
   bech32 = callPackage ../development/python-modules/bech32 { };
 
-  beetcamp = callPackage ../development/python-modules/beetcamp {
-    beets = pkgs.beets.override {
-      python3Packages = self;
-    };
-  };
+  beetcamp = callPackage ../development/python-modules/beetcamp { };
+
+  beets = toPythonModule (pkgs.beets.override { python3Packages = self; });
 
   beewi-smartclim = callPackage ../development/python-modules/beewi-smartclim { };