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

Change-Id: If04a02179d37f5d088881d5d960b3e44b90e1023

.zuul.yaml

@@ -4,8 +4,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3
     check:
       jobs:

setup.cfg

@@ -13,6 +13,7 @@ classifier=
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 keywords = openstack monitoring email
 description-file = README.rst
 home-page = https://github.com/openstack/monasca-notification

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,36,py},pep8,cover
+envlist = py{27,36,37,py},pep8,cover
 minversion = 2.7
 skipsdist = True