In setup.cfg, replace deprecated metadata.description-file

[musicinmybrain]

Use metadata.description_file instead.

Fixes:

/usr/lib/python3.12/site-packages/setuptools/dist.py:472: SetuptoolsDeprecationWarning: Invalid dash-separated options
!!

        ********************************************************************************
        Usage of dash-separated 'description-file' will not be supported in future
        versions. Please use the underscore name 'description_file' instead.

        By 2024-Sep-26, 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.
        ********************************************************************************

!!
  opt = self.warn_dash_deprecation(opt, section)

[mariomulansky]

thanks!