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.
* pythonPackages.yowsup: init at v2.5.2 * python.pkgs.yowsup: fix tests