Fit the StopIteration for py37

In Train we will switch to use py37 for tests.
There is a breaking change between Python 3.6 and 3.7
that is making it an error to raise a StopIteration exception
from a function or generator(https://www.python.org/dev/peps/pep-0479/).
So we need to fix it in code.

Change-Id: I003be4193944bc4176768c488f2f71b58fcbd58d
Closes-Bug: #1837856
Story: #2005924
Task: #34258

.zuul.yaml

@@ -112,7 +112,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

setup.cfg

@@ -15,7 +15,9 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     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,pep8
+envlist = py27,py37,pep8
 skipsdist = True
 
 [testenv]

zaqar/storage/mongodb/claims.py

@@ -38,15 +38,18 @@ LOG = logging.getLogger(__name__)
 def _messages_iter(msg_iter):
     """Used to iterate through messages."""
 
-    msg = next(msg_iter)
-    yield msg.pop('claim')
-    yield msg
-
-    # Smoke it!
-    for msg in msg_iter:
-        del msg['claim']
+    try:
+        msg = next(msg_iter)
+        yield msg.pop('claim')
         yield msg
 
+        # Smoke it!
+        for msg in msg_iter:
+            del msg['claim']
+            yield msg
+    except StopIteration:
+        return
+
 
 class ClaimController(storage.Claim):
     """Implements claim resource operations using MongoDB.