Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Also fix the CI failure due to:
https://docs.python.org/3/whatsnew/3.5.html#pep-479-change-stopiteration-handling-inside-generators

Change-Id: Ica582f99730c5019a1212d4f58d7d854ad2996fd
Story: #2005924
Task: #34254

.zuul.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
       - 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

@@ -1,7 +1,7 @@
 # Python Trove Client
 
 [tox]
-envlist = py36,py27,pypy,pep8
+envlist = py27,py37,pypy,pep8
 minversion = 2.0
 skipsdist = True
 

troveclient/v1/backups.py

@@ -236,13 +236,18 @@ class Backups(base.ManagerWithFind):
         def mistral_execution_generator():
             m = marker
             while True:
-                the_list = mistral_client.executions.list(marker=m, limit=50,
-                                                          sort_dirs='desc')
-                if the_list:
-                    for the_item in the_list:
-                        yield the_item
-                    m = the_list[-1].id
-                else:
+                try:
+                    the_list = mistral_client.executions.list(
+                        marker=m, limit=50,
+                        sort_dirs='desc'
+                    )
+                    if the_list:
+                        for the_item in the_list:
+                            yield the_item
+                        m = the_list[-1].id
+                    else:
+                        return
+                except StopIteration:
                     return
 
         def execution_list_generator():