python3Packages.pyxattr: platforms are the ones xattr supports

This ensures the proper propagation for `lib.meta.availableOn` to work.

(cherry picked from commit d99434c90b)

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

@@ -21,5 +21,6 @@ buildPythonPackage rec {
     meta = with lib; {
       description = "A Python extension module which gives access to the extended attributes for filesystem objects available in some operating systems";
       license = licenses.lgpl21Plus;
+      inherit (pkgs.attr.meta) platforms;
     };
 }