Move mypy and flake8 config to setup.cfg
This fixes bug: https://phabricator.wikimedia.org/T308734
mypy config is ignored when declared in tox.ini. This change introduces a
setup.cfg to declare configs.
While not directly affected, but for consistency reasons, I moved flake8 in there too.
Introducing setup.cfg was a change already planned, and will be adopted in the cookiecutter and existing projects.
@fab knowledge-gaps has the same issue (mypy conf declared in tox.ini).