Replace description-content-type by its underscore name

Since v78.0.0, setuptools no longer accepts options containing
uppercase or dash characters in setup.cfg [1].

[1] https://github.com/pypa/setuptools/blob/main/NEWS.rst#v7800

Closes-Bug: #2104030

Change-Id: Id88b9c73a4cd9511750f38da9393dae3adbc5c1e

Author: amoralej

setup.cfg

@@ -3,7 +3,7 @@ name = python-openstackclient
 summary = OpenStack Command-line Client
 description_file =
     README.rst
-description-content-type = text/x-rst
+description_content_type = text/x-rst
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-openstackclient/latest/