Use pyproject.toml for project metadata, use modern build

[mwtoews]

This PR moves project metadata to pyproject.toml. Both setup.py and setup.cfg are no longer needed, and removed.

Other details:

• Build sdist and wheels using PyPA's build tool, which are uploaded to PyPI (currently only sdist are there)
• Run twine check --strict on artifacts (previously this would have raised a warning)
• Add a tests extra, so that pip install -e .[tests] will also install tests/requirements.txt
• The short description was changed to a better one, from GitHub
• Tool configuration for black and isort are moved from tox.ini to pyproject.toml; the exception is flake8 which does not support pyproject.toml

[FussyDuck]

All committers have signed the CLA.