Check python version to verify requirements, addition to #2457

Hey @probablycorey, i took a stab at adding a python version check onto #2457.

i'm not entirely happy with it at the moment... i don't like that i threw a callback in there, but i couldn't find a way of executing `python` synchronously that wouldn't require me pulling in a ton more code from other npm modules.

i doubt this is perfect, but i gave this a try and figured i'd share what i came up with. i'm still learning :smile:.