Replace deprecated license_file with license_files in setup.cfg

[musicinmybrain]

Fixes:

  /usr/lib/python3.12/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.
          ********************************************************************************

  !!

[asottile]

opted for #453 instead -- but thank you!