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: I2ea9570dcf018c484054d8078f6e5a767b23dd82

setup.cfg

 [metadata]
 name = oslo.limit
 summary = Limit enforcement library to assist with quota calculation.
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/oslo.limit/latest/
-python-requires = >=3.6
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://docs.openstack.org/oslo.limit/latest/
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology