Fix deprecated spelling of license_file in setup.cfg

[cjwatson]

setuptools was producing the following warning:

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

By 2025-Mar-03, 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.