Merge `setup.cfg` into `pyproject.toml`

[BenjaminRodenberg]

pyproject.toml is the recommended format and we are mainly using the setup.cfg for versioneer. Versioneer also supports pyproject.toml. Therefore, moving should be possible. See https://github.com/python-versioneer/python-versioneer?tab=readme-ov-file#vendored-mode.