`setuptools.config.read_configuration` became deprecated

[yarikoptic]

Observed (quietly, no fail) while working on

• BF: use setuptools.errors.OptionError instead of now removed import of distutils.DistutilsOptionError; stop using python 3.8 in CI #98

/home/runner/work/datalad-extension-template/datalad-extension-template/_datalad_buildsupport/setup.py:129: SetuptoolsDeprecationWarning: Deprecated API usage.
!!

        ********************************************************************************
        As setuptools moves its configuration towards `pyproject.toml`,
        `setuptools.config.read_configuration` became deprecated.

        For the time being, you can use the `setuptools.config.setupcfg` module
        to access a backward compatible API, but this module is provisional
        and might be removed in the future.

        To read project metadata, consider using
        ``build.util.project_wheel_metadata`` (https://pypi.org/project/build/).
        For simple scenarios, you can also try parsing the file directly
        with the help of ``configparser``.
        ********************************************************************************

!!
  cfg = read_configuration(
loading translations [en]... done
making output directory... done
Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`.