py3: use the train test template and other updates

Use the unified unit test Zuul template for train, according the
community goal. Also, update the setup.cfg classifiers accordingly.

Update the debug test environment. Adding "debug-py36" is enough;
if a distribution supports 3.7, the "debug" tox environment
can be used.

Finally, keep only the most recent environment in the default
set executed when calling tox.

Story: #2005924
Task: #34244

Change-Id: Iad4a6e933ad416eb67a2f144540fe79e97938bec

.zuul.yaml

@@ -3,9 +3,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3

setup.cfg

@@ -17,8 +17,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [global]
 setup-hooks = pbr.hooks.setup_hook

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py35,py36,py37,pypy,pep8,releasenotes
+envlist = py27,py37,pypy,pep8,releasenotes
 minversion = 2.0
 skipsdist = True
 
@@ -27,8 +27,8 @@ commands = oslo_debug_helper -t saharaclient/tests/unit {posargs}
 basepython = python2.7
 commands = oslo_debug_helper -t saharaclient/tests/unit {posargs}
 
-[testenv:debug-py35]
-basepython = python3.5
+[testenv:debug-py36]
+basepython = python3.6
 commands = oslo_debug_helper -t saharaclient/tests/unit {posargs}
 
 [testenv:cover]