In Arch, /usr/bin/python means python3. So you'll need to replace all mentions of that path with /usr/bin/python2. Actually likewise, s|/usr/bin/env python|/usr/bin/env python2|g. See https://aur.archlinux.org/packages/pybitmessage-git/
In fact you'll also need to set an absolute path for translations. I haven't run your script myself yet, but I suspect that you'll also need to do some fancy installation of icons, as the pybitmessage-git AUR package does. I'd strongly recommend comparing the output that you get from the two packages and making sure they're pretty darn similar.