Update Python 3 test runtimes for Train

One of train goals to have consistent python3 jobs.

https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: If72309322644ea54fbf3a07ecbaa1bcf09ff9dba

setup.cfg

@@ -16,6 +16,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages = ironicclient

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py27,pep8,pypy
+envlist = py3,py27,pep8,pypy
 skipsdist = True
 
 [testenv]

zuul.d/project.yaml

@@ -4,8 +4,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
-      - openstack-python37-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3