OMERO.py: src flake8 (part I)

See https://trello.com/c/vnojR45Q/43-flake8-omeropy. To be included post- 5.0.3 release, this PR is the first of a series dedicated to `flake8` the `src` folder of `OmeroPy`.

To review this PR:

- check Travis is green and that the `flake8` is run on the files modified by this PR in the Python components
- check the integration tests are still passing

/cc @ximenesuk