Move package setup to pyproject.toml

[dhimmel]

References:

• https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html
• https://packaging.python.org/en/latest/specifications/declaring-project-metadata/
• https://peps.python.org/pep-0621/
• https://ini2toml.readthedocs.io/en/latest/index.html

[dhimmel]

Noting the warning that:

Support for declaring project metadata or configuring setuptools via pyproject.toml files is still experimental and might change (or be removed) in future releases.

However, it's working great locally with pip 21.2.3 in python 3.9 (and on CI).