Migrate pythonapi's setup.cfg to pyproject.toml

[vanderhe]

I also reorganized the flat-/src-layout hybrid as a standard src-layout project and renamed the project from 'pythonapi' to 'dftbplus', which should not have any side-effects e.g. for the dftbplus-python-feedstock.

In any case, the recipes are outdated (still referring to setup.py), so a supplementary PR in dftbplus/recipes is needed.