configure flake8 in pyproject.toml

[jGaboardi]

Following #415/#420, we should keep an eye on progress/changes with the flake8 functionality in pyproject.toml files. Currently, flake8 is not configurable within pyproject.toml files, which leads to the need for keeping setup.cfg around.

There seem to be several solutions, but I was not able to get any to work properly.

• flake8-pyproject
• pyproject-flake8
• etc.

Once this is worked out, setup.cfg can be removed.