Update python testing as per zed cycle teting 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: Iba5074ea6f981a7527e86cfc98edd1ed7dd3086f

setup.cfg

@@ -10,7 +10,7 @@ project_urls =
     Bug Tracker = https://bugs.launchpad.net/nova/
     Documentation = https://docs.openstack.org/nova/
     Source Code = https://opendev.org/openstack/nova
-python_requires = >=3.6
+python_requires = >=3.8
 classifiers =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -20,8 +20,6 @@ classifiers =
     Operating System :: POSIX :: Linux
     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