Remove deprecated license_file parameter

[stevejalim]

Noticed this while packaging 0.7.0:

         ********************************************************************************
        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.
        ********************************************************************************

[stevejalim]

Heads up @pmac @robhudson - we'll need to do this across all our packages. What isn't clear is whether in-the-wild packages will break, or if this just stops us doing a fresh release until we make the change