Change import grouping style recommendation to follow PEP8

One great thing about the style guide is it's simplicity: follow the PEP's recommendations, specially PEP8.

This pull request simply suggests adhering to PEP8 on import grouping.
The tools we use to write code probably do.
I can say that rope (http://rope.sourceforge.net) follows this grouping in it's "organize imports" refactoring.