Switch setup.py to pyproject.toml?

[MichaelClerx]

pip now complains if you try to install with

pip install -e .

DEPRECATION: Legacy editable install of pints==0.5.1 from file:///home/michael/dev/pints (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at pypa/pip#11457