Reorganise setup.cfg

[bsipocz]

As it was mentioned in #279 (comment) it would be useful to be able to rely on setup.cfg for dependencies, including optional and test dependencies.

(e.g. pillow was added as a test dependency in #276. While it's being picked up when someone runs the full tox workflow, it would be nicer to be able to say python -m pip install -e .[test] too to pick up any of those dependencies.)