invalid dash-separated key in setup.cfg

The use of the key "description-file" in `setup.cfg` should be re-named to "description_file" due to issues with modern setuptools.

```
          raise InvalidConfigError(
      setuptools.errors.InvalidConfigError: Invalid dash-separated key 'description-file' in 'metadata' (setup.cfg), please use the underscore name 'description_file' instead.
```