OmeroPy unit tests flake8 (rebased onto develop)



This is the same as gh-2224 but rebased onto develop.

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This PR fixes the flake8 output on the remaining Python unit tests under `components/tools/OmerPy/test/unit`.

To test this PR:
- check Travis is green (`flake8` is now run on the entire `test/unit` folder)
- check the unit tests are still passing in the next run of http://ci.openmicroscopy.org/job/OMERO-5.0-merge-integration-python/lastCompletedBuild/testReport/