Removed unused imports (or marked them flake8 ignore).

It's kind of annoying to put `# NOQA` in the all the `__init__.py` files where we have convenience imports.

Putting this up for review to get opinions on that.

Aternatives:
* use `__all__` as suggested here: https://bugs.launchpad.net/pyflakes/+bug/1178905
* Ignore all `__init__.py` files (in which case we won't get flake8 checking for the rest of the errors either)
* your idea here