refactor(run_tests): python 2/3 compatibility
refactor(run_tests): python 2/3 compatibility

That way, it will run on travis as well, which comes with python 2
by default, and just doesn't have python 3 pre-installed.
It would probably take to much time to install, especially considering
it's super easy to have to script work in both worlds.

sru/clap-rs,daboross/clap-rs,cstorey/clap-rs,clap-rs/clap,sru/clap-rs,clap-rs/clap,darayus/clap-rs,grossws/clap-rs,kbknapp/clap-rs,Vinatorul/clap-rs,daboross/clap-rs,Vinatorul/clap-rs,darayus/clap-rs,wezm/clap-rs,cstorey/clap-rs,kbknapp/clap-rs,nelsonjchen/clap-rs,wezm/clap-rs,cstorey/clap-rs,sru/clap-rs,grossws/clap-rs,nelsonjchen/clap-rs,daboross/clap-rs,grossws/clap-rs,Vinatorul/clap-rs,clap-rs/clap,wezm/clap-rs,darayus/clap-rs,nelsonjchen/clap-rs