Drop python3.6 support in testing runtime

In Zed cycle testing runtime, we are targetting to drop the
python 3.6 support, project started adding python 3.8 as minimum,
example nova:
- https://github.com/openstack/nova/blob/56b5aed08c6a3ed81b78dc216f0165ebfe3c3350/setup.cfg#L13

Change-Id: I1e0d0a43cd166e6cfd4be7ca0f426e9fc3625a28

.zuul.yaml

@@ -15,7 +15,7 @@
             voting: false
         - cinder-tox-bandit-baseline:
             voting: false
-        - openstack-tox-functional-py36:
+        - openstack-tox-functional-py38:
             irrelevant-files: &functional-irrelevant-files
               - ^.*\.rst$
               - ^cinder/locale/.*$

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/cinder/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,8 +17,6 @@ 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
 project_urls: