Simplify build infrastructure

[eirrgang]

My extensions for PEP517/518 builds are a bit arcane. They can be made more normative through better-documented setuptools extension mechanisms, I think. Instead of our current setup.py and _custom_build/backend.py, I will probably just make a setuptools plugin for gromacs software client support (which will also be useful for the gmxapi package, itself). See https://setuptools.pypa.io/en/latest/userguide/extension.html

[eirrgang]

Suggest migrating to scikit-build-core. See, for instance, https://gitlab.com/gromacs/gromacs/-/merge_requests/3531

Version 0.3 adds experimental support for editable installs.

I don't think we could expect versioningit to work with scikit-build-core (it is explicitly a setuptools plugin), but the new setuptools_scm wrapper might work? See https://scikit-build-core.readthedocs.io/en/latest/configuration.html#dynamic-metadata-wip