Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: I1bd306698a0b2afd1b824eda3389fcc3f919a1fb

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/kayobe/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 license = Apache License, Version 2.0
 classifier =
     Environment :: OpenStack
@@ -18,8 +18,6 @@ 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