diffuse: do not use top-level python packages

pkgs/applications/version-management/diffuse/default.nix

···
-{ stdenv, fetchurl, python, pygtk, makeWrapper }:
+{ stdenv, fetchurl, python27Packages, makeWrapper }:
 
-stdenv.mkDerivation rec {
+let
+  inherit (python27Packages) pygtk python;
+in stdenv.mkDerivation rec {
   version = "0.4.8";
   name = "diffuse-${version}";