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. Moving the py36 job to py38 based but to run on
ubuntu focal as c8s does not seems to have py38.
Also updating the python classifier also to reflect the same.

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

Change-Id: Ic26a360d2bb09fa6622d1acaa5021c5afbc70240

.zuul.yaml

@@ -21,13 +21,13 @@
       - ^cinderclient/tests/unit/.*$
 
 - job:
-    name: python-cinderclient-functional-py36
+    name: python-cinderclient-functional-py38
     parent: python-cinderclient-functional-base
-    # need to specify a platform that has python 3.6 available
-    nodeset: devstack-single-node-centos-8-stream
+    # need to specify a platform that has python 3.8 available
+    nodeset: openstack-single-node-focal
     vars:
-      python_version: 3.6
-      tox_envlist: functional-py36
+      python_version: 3.8
+      tox_envlist: functional-py38
 
 - job:
     name: python-cinderclient-functional-py39
@@ -42,16 +42,16 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-python3-yoga-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:
       jobs:
-        - python-cinderclient-functional-py36
+        - python-cinderclient-functional-py38
         - python-cinderclient-functional-py39
         - openstack-tox-pylint:
             voting: false
     gate:
       jobs:
-        - python-cinderclient-functional-py36
+        - python-cinderclient-functional-py38
         - python-cinderclient-functional-py39

releasenotes/notes/drop-python-3-6-and-3-7-fe2dc753e456b527.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.
+

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-cinderclient/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -18,8 +18,6 @@ classifier =
     Programming Language :: Python
     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
 

tox.ini

@@ -98,7 +98,7 @@ setenv =
 # TLS (https) server certificate.
 passenv = OS_*
 
-[testenv:functional-py36]
+[testenv:functional-py38]
 deps = {[testenv:functional]deps}
 setenv = {[testenv:functional]setenv}
 passenv = {[testenv:functional]passenv}