Drop python3.6/3.7 support in testing runtime

In Zed cycle testing runtime, we are targetting to drop the
python 3.6/3.7 support, project started adding python 3.8 as minimum,

example nova:
- 56b5aed08c/setup.cfg (L13)

Also indicates that we support python 3.9.

Change-Id: I1bdec397b1ac563b09174763c20e8a91e5f86143

setup.cfg

@@ -5,14 +5,14 @@ author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 summary = Manage dynamic plugins for Python applications
 home_page = https://docs.openstack.org/stevedore/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     License :: OSI Approved :: Apache Software License
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython
     Intended Audience :: Developers