Update pyproject.toml to PEP621

This is an issue to discuss the packaging related files, mainly the pyproject.toml.
Since PEP517 et PEP621, it should be possible to put everything in this toml file.
A very simple setup.py file can stay in the repo for backward compatibility.
The build-requirement section ensure the latest compatible setuptools is installed before trying to build the wheel.

But it will be necessary to test if the package still installs correctly on old python versions (3.6 and 3.7).
Also will update metadata according to #50 (closed) decisions.