Warn no return doesn't understand six.reraise

As far as I can tell, mypy doesn't currently have any special knowledge of six, so it doesn't understand that six.reraise raises an exception. If mypy having special understanding of six seems reasonable, we should add this.

Mypy also needs to be taught about other standard library functions which don't return, like sys.exit.

Marking this as high priority this seems to be a blocker for enabling --warn-no-return by default, which we'd like to do.