Migrate metadata from setup.py and setup.cfg to pyproject.toml

[cclauss]

This pull request was created using https://pypi.org/project/setuptools-py2cfg plus manual modifications.

https://setuptools.pypa.io/en/latest/userguide/quickstart.html#basic-use

We recommend users expose as much as possible configuration in a more declarative way via the pyproject.toml or setup.cfg, and keep the setup.py minimal with only the dynamic parts (or even omit it completely if applicable).

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Migrate to pyproject.toml using ini2toml to do the file conversion and then run validate-pyproject to validate the results.

• Currently flake8 is not compatible with pyproject.toml so put its config in a dedicated .flake8 file.

https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html