Use a pyproject.toml instead of setup.py

[Ian2012]

Currently, when installing the harmony tutor plugin on python 3.12 the following deprecation warning appears:

DEPRECATION: tutor-contrib-harmony-plugin is being installed using the legacy 'setup.py install' method, because it does not have a 'pyproject.toml' and the 'wheel' package is not installed. pip 23.1 will enforce this behaviour change. A possible replacement is to enable the '--use-pep517' option. Discussion can be found at https://github.com/pypa/pip/issues/8559

We need to use a pyproject.toml file as tutor does: https://github.com/overhangio/tutor/blob/master/pyproject.toml