Move `pytest` configuration from `setup.cfg` to `pytest.ini`, `pyproject.toml` or `tox.ini`

[Squadula]

pytest documentation:

Warning

Usage of setup.cfg is not recommended unless for very simple use cases. .cfg files use a different parser than pytest.ini and tox.ini which might cause hard to track down problems. When possible, it is recommended to use the latter files, or pyproject.toml, to hold your pytest configuration.