setup.cfg: Resolve warning

setuptools is now emitting the following warning when building nova:

  UserWarning: Usage of dash-separated 'python-requires' will not be
  supported in future versions. Please use the underscore name
  'python_requires' instead

Do as it says. While we're here, we update a few other options to use
their new variants and add some additional metadata. This leaves one
final warning:

  Warning: Unknown distribution option: 'requires_python'

That unfortunately cannot be fixed here and requires a separate fix in
pbr.

Change-Id: I859bc89bc1ee80652f471c3463ef6562d720263b
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

setup.cfg

@@ -1,13 +1,18 @@
 [metadata]
 name = nova
 summary = Cloud computing fabric controller
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/nova/latest/
-python-requires = >=3.6
-classifier =
+author_email = openstack-discuss@lists.openstack.org
+url = https://docs.openstack.org/nova/latest/
+project_urls =
+    Bug Tracker = https://bugs.launchpad.net/nova/
+    Documentation = https://docs.openstack.org/nova/
+    Source Code = https://opendev.org/openstack/nova
+python_requires = >=3.6
+classifiers =
+    Development Status :: 5 - Production/Stable
     Environment :: OpenStack
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators