Converted the setup.cfg settings to a format that pyproject.toml can utilize. Consequently, setup.cfg was removed, as everything is now handled in the pyproject.toml; "source" arrays have had their values swapped as necessary. The only discrepancies in the migration are:

• metadata.group was removed. This simply isn't a valid option in pyproject.toml, and this info can be extrapolated from project.classifiers.
• options.data_files was removed. This is considered a discouraged & largely unsupported option, with the listed files already getting caught by tool.setuptools.package-data anyway.

The only other change made is fixing the mypy section of pyproject.toml; turns out its syntax was incorrect this whole time. Unsure if explicit tests are necessary in this instance, as they're already validated by the buildsystem itself.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation