In setup.cfg, avoid deprecated license_file

[musicinmybrain]

Use license_files instead.

https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

Fixes warnings like:

!!

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

        This deprecation is overdue, please update your project and remove deprecated
        calls to avoid build errors in the future.

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

!!