FAQ: add link for how to switch to Python 3 checker automatically.

I added a link to [my Gist](https://gist.github.com/eddieantonio/c096c780cf09c938e031) on how to switch to the Python 3 syntax checker automatically based on the shebang line. Let me know if this is an appropriate addition to the FAQ; I know that I frequently go between modifying Python 2 and Python 3 sources, so this happens to be _really_ convenient for me—perhaps this is true for other people as well.