Vim Syntastic: automatically switch between python2 and python3 syntax checkers.

python.vim

" Place this file in {VIMHOME}/after/ftplugin/python.vim
" Must have syntastic >= 3.5.0-29

" Use Python 3 when the shebang calls for it.
let b:syntastic_python_python_exec = 
  \ syntastic#util#parseShebang()['exe']  =~# '\m\<python3'
  \ ? 'python3'
  \ : 'python'

three.py

#!/usr/bin/env python3

def generator():
    yield from range(10)

two.py

#!/usr/bin/env python

print "This is Python 2"