PEP 621: Migrate to pyproject.toml

[cclauss]

Migrate to pyproject.toml using ini2toml to do the file conversion and running validate-pyproject in pre-commit to validate the results.

• setup.cfg: kewords --> keywords See pre-commit: Fix typos using codespell #141
• Comment out tool.setuptools.test-suite which was deprecated in setuptool v41.5.0 and simply run pytest
• Currently flake8 is not compatible with pyproject.toml so put its config in a dedicated .flake8 file.
• Test with pipx install --force git+https://github.com/cclauss/[email protected]

From validate-pyproject README:

With the approval of PEP 517 and PEP 518, the Python community shifted towards a strong focus on standardisation for packaging software, which allows more freedom when choosing tools during development and make sure packages created using different technologies can interoperate without the need for custom installation procedures.

This shift became even more clear when PEP 621 was also approved, as a standardised way of specifying project metadata and dependencies.

validate-pyproject was born in this context, with the mission of validating pyproject.toml files, and making sure they are compliant with the standards and PEPs.

[fsouza]

Now that setup.py is gone, how should we release? Can you also update the workflow declared in upload-to-pypi.yaml?

[fsouza]

Now that we have pyproject perhaps we can migrate to poetry to manage dev dependencies too. No need to do it as part of this PR though.

[fsouza]

Thanks for contributing! I'm going to merge and migrate to poetry in a follow-up PR. That should take care of uploading to PyPI too.