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: I298bf00067bd877e4d29cddd15512b9e7fda3bf3

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack Documentation
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/openstack-doc-tools/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,8 +17,8 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =