Easiest solution:
Add this to you .vimrc
let g:syntastic_python_python_exec="python3"
let g:syntastic_python_checkers = ['python']
This is the straightforward solution to switch to python3.
Add this to you .vimrc
let g:syntastic_python_python_exec="python3"
let g:syntastic_python_checkers = ['python']
This is the straightforward solution to switch to python3.