Add a pytproject.toml file

[InnocentBug]

This hopefully defines a good build environment for plum-dispatch.

[InnocentBug]

Ok, so I researched it a bit.
setuptools does not fully support integration with pyproject.toml. See here for their discussion on it. pypa/setuptools#1688

However, what they propose is to eliminate setup.py entirely. Which solves our issue, because if you remove the setup.py users are forced to install via pip directly.

We could specify all the config of setup.py in setup.cfg and make the build environment in pyproject.toml. The only thing that wouldn't work in the current scenario is the automatic version determination via minivers.
So if move back to manual specification of the version, I can try it out.

I am not really a fan of this, but maybe it is lesser evil.

[pabloferz]

It seems that there are efforts to deprecate installing via setup.py anyway. In case we do that and leave only a pyproject.toml and setup.cfg, we should be able to use https://github.com/pypa/setuptools_scm/ as alternative to miniver, as it looks to be compatible with pyproject.toml (and seems to be the closest to an official way of doing this)

[InnocentBug]

Good idea to switch to the setuptools_scm.

I implemented it and could drop the setup.py entirely.
A bit ironic, because this is what I originally wanted, but pyproject.toml seems to be the future.