Replace 'raise StopIteration' with 'return'

With PEP 479, the behaviour of StopIteration is changing. Raising it to
stop a generator is considered incorrect and from Python 3.7 this will
cause a RuntimeError. The PEP recommends using the return statement.

More details: https://www.python.org/dev/peps/pep-0479/#examples-of-breakage

Change-Id: Ib27581fccbbf14c082fb919d8b6edea1ac83e3c0

oslo_messaging/_drivers/pool.py

@@ -117,7 +117,7 @@ class Pool(object):
                 _, item = self._items.pop()
                 yield item
             except IndexError:
-                raise StopIteration
+                return
 
     @abc.abstractmethod
     def create(self):

oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py

@@ -170,7 +170,7 @@ class RoutingTable(object):
         hosts, tm_original = self._get_hosts(target_key)
         for host in itertools.cycle(hosts):
             if self._is_target_changed(target_key, tm_original):
-                raise StopIteration()
+                return
             yield host