Create pyproject.toml for project metadata and build dependencies

[mwtoews]

This PR add pyproject.toml to describe the project metadata (docs) and build dependencies. Keep setup.py for now, as some environments expect to see this file. Also, require Python 3.7+, as noted in recent commit. Replace it with Python 3.9 in README.rst (even though this is two versions behind the latest Python 3.11).

The sdist and whl packages can simply be built and checked using:

pip install build twine
rm -rf dist
python -m build
twine check --strict dist/*