python-common: add a pyproject.toml file

In order to support the current python packaging standards we need
to have a pyproject.toml [1] file. This file defines the project's
metadata and build tool.

For continuity, I have left setuptools in place as the build backend
so the existing steup.py is still in play. I also experimented with
flit as a back-end. Flit seemed to work OK but I was a bit unsure
how distro support for it would be when we started to roll out this
option. Thus, to be safe I decided to stay with setuptools for now.

[1] https://packaging.python.org/en/latest/guides/writing-pyproject-toml/

See Also:
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/
https://flit.pypa.io/en/stable/pyproject_toml.html

Signed-off-by: John Mulligan <jmulligan@redhat.com>