setup.cfg: Replace dashes with underscores
Setuptools v54.1.0 introduces a warning that the use of dash-separated options in 'setup.cfg' will not be supported in a future version [1].
Get ahead of the issue by replacing the dashes with underscores. Without this, we see 'UserWarning' messages like the following on new enough versions of setuptools:
UserWarning: Usage of dash-separated 'description-file' will not be
supported in future versions. Please use the underscore name
'description_file' instead