How can I set the Python max allowed line length to 120 in Syntastic for Vim?

This is now configured globally in ~/.config/flake8 in linux and ~/.flake in windows. The file can look like this.

[flake8]
max-line-length = 120

Have a look at their documentation for per project settings: http://flake8.readthedocs.io/en/latest/user/configuration.html#project-configuration