Update setup.cfg and tox.ini

Add Python 3.9 to the list of supported Pythons and replace
dashes (which are deprecated) 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: I12cc6206ebfe06b7cf3a50ea59bf1b55a3ff7303

setup.cfg

@@ -2,11 +2,11 @@
 name = refstack-client
 version = 0.1.1
 summary = Tempest test wrapper and result uploader for refstack
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://refstack.openstack.org
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://refstack.openstack.org
 classifier =
     Environment :: OpenStack
     Intended Audience :: Developers
@@ -21,6 +21,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py35,py36,py38,py27
+envlist = pep8,py3,py27
 minversion = 3.18
 skipsdist = True