adhere to PEP008 by using absolute imports

See https://www.python.org/dev/peps/pep-0008/#imports.

This is also how it's done in all of Python's standard library.

@jbweston and @anton-akhmerov is this good to merge?

This seems fine, but in general I'm not really in favor of refactors that have no tangible benefit.

i.e. touching the code in many places invites mistakes to be made, makes git blame less useful etc.

merged

mentioned in issue #125 (closed)