Migrate to pyproject.toml

[PMeira]

Since the hard part of the build is now in dss_python_backend, this is a pure Python package (same as ODD.py). We should adopt pyproject.toml instead of setup.py.

[PMeira]

Depending on how long we delay that, we may need to update setup.cfg:

/opt/python/cp39-cp39/lib/python3.9/site-packages/setuptools/config/setupcfg.py:293: _DeprecatedConfig: Deprecated config in `setup.cfg`
!!

        ********************************************************************************
        The license_file parameter is deprecated, use license_files instead.

        By 2023-Oct-30, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
        ********************************************************************************