pythonPackages.yowsup: disable python2 build

I'm not exactly sure why, but it seems as the python2 build
of yowsup is breaking.
see https://github.com/tgalal/yowsup/issues/2325

It seems to be recommended to use python3 for building and disable
the usage of python2 which fixed the build.

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

@@ -1,10 +1,16 @@
-{ buildPythonPackage, stdenv, fetchFromGitHub, six, python-axolotl, pytest }:
+{ buildPythonPackage, stdenv, fetchFromGitHub, six, python-axolotl, pytest
+, isPy3k
+}:
 
 buildPythonPackage rec {
   name = "${pname}-${version}";
   pname = "yowsup";
   version = "2.5.2";
 
+  # python2 is currently incompatible with yowsup:
+  # https://github.com/tgalal/yowsup/issues/2325#issuecomment-343516519
+  disabled = !isPy3k;
+
   src = fetchFromGitHub {
     owner = "tgalal";
     repo = "yowsup";