Use `pyproject.toml` for PEP 517 / PEP 518 metadata

[AA-Turner]

Currently this project uses a common setup.py for each of its packages.

Please consider migrating to the standardardised pyproject.toml, as installing by running ./setup.py has been deprecated for several years.

The minimum required is generating a pyproject.toml file that contains the below:

[build-system]
requires = ["setuptools>=42"]
build-backend = "setuptools.build_meta"

This is all that's required to move to standardised build metadata, away from the legacy setup.py approach.

---

You could also consider using standard project metadata, via the [metadata] table, and a different build backend, such as flit-core, which is smaller and a little faster. See e.g. https://flit.pypa.io/en/stable/pyproject_toml.html#new-style-metadata.

A