Support PEP 621

[virtuald]

https://www.python.org/dev/peps/pep-0621/ standardizes storing project metadata in pyproject.toml. We basically do the same thing already, so it would involve reading a different key and deprecating the old mechanism.

[auscompgeek]

PEP 621 has been provisionally accepted: https://discuss.python.org/t/pep-621-round-3/5472/109

[auscompgeek]

Looks like setuptools supports PEP 621 natively since v61: https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6100

(note: setuptools v59.7 requires Python 3.7+)