Use pyproject.toml instead of setup.cfg

[ianthomas23]

This PR replaces use of setup.cfg with the equivalent pyproject.toml for PEP 517 compliance.

The setup.py is kept for backward compatibility using the same form as used by various projects in the Jupyter ecosystem, e.g. https://github.com/jupyterlab/jupyterlab/blob/main/setup.py

I have added some extra classifiers for "Jupyter" and also defined the development status as "Production/Stable" as that is what I figure it is. I have removed classifiers for version of Python prior to 3.8 as they have reached end of life (https://devguide.python.org/versions).

[ianthomas23]

I have also compared the built sdist and wheel before and after this PR to confirm they are equivalent, and confirmed manually that the installed wheel works with JupyterLab 3 and 4, Notebook 6 and 7, and NbClassic.

[martinRenou]

Thanks for doing this! Let's continue discussing the matplotlib optional dependency in #4