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: I532be460dc673548be9ed3b540e9feb8ca832fbe
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

setup.cfg

@@ -1,12 +1,12 @@
 [metadata]
 name = tempest
 summary = OpenStack Integration Testing
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/tempest/latest/
-python-requires = >=3.6
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://docs.openstack.org/tempest/latest/
+python_requires = >=3.6
 classifier =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -48,4 +48,3 @@ tempest.cm =
     run = tempest.cmd.run:TempestRun
 oslo.config.opts =
     tempest.config = tempest.config:list_opts
-