Move setuptools configuration into pyproject.toml

[seanh]

Since version 61.0 (March 24th 2022) setuptools now supports pyproject.toml and its docs advertise pyproject.toml as the first/preferred place for setuptools config. If we move the setuptools config from setup.cfg into pyproject.toml then if we also remove the pycodestyle config from setup.cfg (#52) we could get rid of the setup.cfg file entirely and have one less root file.