Add setup.cfg at the source root of OMERO.py (rebased onto develop)



This is the same as gh-2370 but rebased onto develop.

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Similar to what was done for the OMERO.py `test/` folder, this PR:
- adds a main setup.cfg file at the root of `components/tools/OmeroPy/src` 
- explicitly lists all non-flake8'd files/directories
- uses this configuration file in the `tools/travis-build` Python target.
- performs trivial code validation on a couple of Python files.

To test this PR, check Travis is green and/or run `flake8 -v components/tools/OmeroPy/src` locally. /cc @ximenesuk