Use pyproject.toml and fix coverage

[vringar]

As described in the 4.0.0 release notes pytest-cov removed support for multiprocessing.
Instead of adding an additional config file, it seems seems better to migrate from the old setup.cfg to the new pyproject.toml and configure coverage there according to this documentation.

By migrating from the setup.cfg to pyproject.toml for all the other configuration we keep the amount of config files constant.

Both mypy and isort support and recommend pyproject.toml.