[python] Generate `pyproject.toml` for our packaging, slowly deprecate `setup.py`

The python world is moving from `setup.py` -> `pyproject.toml`, so we should go with the tide. For our new package generations, I think we can switch entirely over to `pyproject.toml` instead of `setup.py`. This will gradually shift the makeup of our packages to `pyproject.toml`. Please make note to publicize this at least in the Python team's chat