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

[1] https://github.com/pypa/setuptools/commit/a2e9ae4cb

Change-Id: I83fc7f428b179e038b57e8c2f0bbc7be6879cacb