pythonPackages.xgboost: only with Python 3.x

pkgs/top-level/python-packages.nix

@@ -30140,6 +30140,9 @@ EOF
     propagatedBuildInputs = with self; [ scipy ];
     buildInputs = with self; [ nose ];
 
+    # Cannot be installed with Python 2.x, most likely due to the patch below.
+    disabled = !isPy3k;
+
     postPatch = ''
       cd python-package