Switch python3 versions of test jobs to match Train PTI

In Train, we will use python 3.6 and 3.7 for python3 runtime
in our gate jobs [1]. This commit also adds python 3.7.

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

Depends-On: https://review.opendev.org/#/c/667039/
Change-Id: Iac3770f6d833340579056998b5fa9c71df5b8660

.zuul.yaml

@@ -3,8 +3,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -17,6 +17,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Topic :: System :: Monitoring
 
 [global]

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.3.1
-envlist = py36,py27,pep8
+envlist = py37,py36,py27,pep8
 skipsdist = True
 
 [testenv]