setup.cfg should use `description_file` key to move away from deprecated dash-style

https://github.com/click-contrib/click-completion/blob/84509aaec4d9a377ad4aa0289056013666b86ba0/setup.cfg#L2

https://github.com/pypa/setuptools/blob/3106af0512fe67464a8b5e7524c07fddf7717660/setuptools/dist.py#L499-L509