Add the penstack-python3-train-jobs template

[1]https://governance.openstack.org/tc/resolutions/20181024-python-update-process.html
[2]https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: I535a62712b915cfc6a1e759301ea0fa28ffb9389
Signed-off-by: song baisen <songbaisen@szzt.com.cn>
Co-Authored-By: tangzhuo <ztang@hnu.edu.cn>, zhiyuan_cai <luckyvega.g@gmail.com>

.zuul.yaml

@@ -40,7 +40,7 @@
     templates:
       - openstack-python-jobs-neutron
       - openstack-python36-jobs-neutron
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - check-requirements
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

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 =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py27,pypy,pep8
+envlist = py36,py37,py27,pypy,pep8
 skipsdist = True
 
 [testenv]