I added the changelog entry and merged the branch.

Before you merge: even if this is mostly a developer-facing change, I think a brief changelog entry would be good.

I did a review. The majority of changes fixes what I would consider "formatting errors", so this is nice.

Pull request: https://bitbucket.org/jendrikseipp/downward/pull-requests/139

Previously, we checked for whitespace "errors" in the translator and we checked for unused imports and unused local variables in the driver and translator. Going forward, I'd like to check for these things and some additional style conventions in all Python files in our repository with flake8. 

As we've done for similar issues in the past, this issue only makes the necessary changes to our codebase for the tests to pass and doesn't include the tests themselves.