When installing python-powerline-git on Arch, opening vim produces an error message by default that needs to be skipped every time. By adding let g:pymcd_powerline="py3" to the .vimrc this can be fixed for a single user, and by adding that line atop /usr/share/vim/vimfiles/plugin/powerline.vim it can be done system-wide.
Because the python3 and python2 packages conflict each other, I think the py3 PKGBUILDs should add that setting on their own, since python2 seems to be the default.
Or the vim loading mechanism should try both before failing