Migrate from `setup.py` to `pyproject.toml`

[fleighton]

Running setup.py directly is considered deprecated, so we should move to using a pyproject.toml file to define the package setup.

See:

• https://setuptools.pypa.io/en/latest/userguide/quickstart.html#setuppy-discouraged
• https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html