pythonPackages.setuptools: specify priority

Both python3 and setuptools come with easy-install. For some magic
reason this hasn't caused any collisions yet, but it does with #17428.
We hereby prioritize the version that comes with setuptools.

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

@@ -28,5 +28,6 @@ stdenv.mkDerivation rec {
     homepage = http://pypi.python.org/pypi/setuptools;
     license = with lib.licenses; [ psfl zpt20 ];
     platforms = platforms.all;
+    priority = 10;
   };
 }