pythonPackages: Add a callPackage attribute.

This is to allow for easy overriding using <some_pkg>.override <overrides> and
might be used by other python modules not directly in pythonPackages.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/top-level/python-packages.nix

@@ -13,6 +13,8 @@ pythonPackages = python.modules // rec {
 
   # helpers
 
+  callPackage = pkgs.lib.callPackageWith (pkgs // pythonPackages);
+
   buildPythonPackage = import ../development/python-modules/generic {
     inherit (pkgs) lib;
     inherit python wrapPython setuptools recursivePthLoader offlineDistutils;