Add python37 job

Since we've entered Train and the runtime proposed include py37[1],
we need to include it in gate as well as local env.
This patch adds the support for the same.

Update the constraints url For more detail, see [2]

[1] https://governance.openstack.org/tc/reference/runtimes/train.html
[2] http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html

Change-Id: I8c512c95322cde5e1163ee35154a48826871cb36

.zuul.yaml

@@ -2,5 +2,5 @@
     templates:
       - check-requirements
       - openstack-python-jobs-horizon
-      - openstack-python36-jobs-horizon
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3

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
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py27,py36,pep8
+envlist = py27,py36,py37,pep8
 skipsdist = True
 
 [testenv]
@@ -8,7 +8,7 @@ usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = /bin/bash run_tests.sh -N --no-pep8 {posargs}