Add py38 package metadata
Add py38 package metadata

Now that we are running the Victoria tests that include a
voting py38, we can now add the Python 3.8 metadata to the
package information to reflect that support.

Change-Id: Idfa3b9d4d41a3cb456fe9f24872dd6cdd17299bf

openstack/designate-dashboard,openstack/designate-dashboard,openstack/designate-dashboard