Convert package to PEP621 pyproject.toml

[miketheman]

https://peps.python.org/pep-0621/
https://packaging.python.org/en/latest/specifications/declaring-project-metadata/#declaring-project-metadata

Some considerations need to be taken for current setup.py dynamic attributes from __about__.py:

readme_renderer/setup.py

Lines 21 to 23 in c6f1eab

	with open(base_dir.joinpath("readme_renderer", "__about__.py")) as f:
	about = {}
	exec(f.read(), about)

readme_renderer/setup.py

Lines 30 to 37 in c6f1eab

	name=about["__title__"],
	version=about["__version__"],
	description=about["__summary__"],
	long_description=long_description,
	license=about["__license__"],
	url=about["__uri__"],
	author=about["__author__"],
	author_email=about["__email__"],

Some inspiration could be drawn from here: https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html#dynamic-metadata

Other values might be better served as static metadata.

[di]

Resolved by #293.