Update supported python versions

- Python 3.5 support was already removed from Tempest[1] so we can
  remove it.
- Add Python 3.8, 3.9 and 3.10 ecause these have been part of tested
  runtimes for a few cycles and are actually used in testing.

[1] b0a8057fbbf08ff072bfb6e836535f090fd6efc7

Change-Id: I39b822ef294f673adfb35e5d45672ad1f9347944

setup.cfg

@@ -6,6 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = http://docs.openstack.org/developer/heat-tempest-plugin
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -14,9 +15,11 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
 
 keywords =
     setup