Well, Luca beat me and added a patch to fix the build an hour ago (Thanks!). You can install it from KDE:Extra now already: http://software.opensuse.org/download.html?project=KDE%3AExtra&package=kdevelop5-plugin-python3 (it will probably take a few days until this reaches Tumbleweed...) I haven't tested it myself though, as I don't have a TW installation currently.