build: use pyproject.toml

[jolars]

This PR refactors the current use of setup.cfg, setup.py, and requremtents.txt into pyproject.toml.

[mathurinm]

I think you also need to specify where it is defined, like we do here : https://github.com/scikit-learn-contrib/skglm/blob/main/pyproject.toml#L36

[mathurinm]

yep, so far it's thinking this is version 0.0.0:

      Successfully uninstalled libsvmdata-0.5.dev0
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
celer 0.7.3.dev0 requires libsvmdata>=0.3, but you have libsvmdata 0.0.0 which is incompatible.
Successfully installed libsvmdata-0.0.0

[jolars]

Of course, thanks! Fixed it now.