Fix py37 compatibility

Unit tests are failing under python3.7.
Generators which explicitly raise StopIteration can generally be
changed to simply return instead. This will be compatible with
all existing Python versions.

PEP Documentation for this change:
https://www.python.org/dev/peps/pep-0479/

Change-Id: I4ae2049d8a2469d0a37077bdc722481e68d7cc49
Closes-Bug: #1814890

magnumclient/common/httpclient.py

@@ -406,7 +406,10 @@ class ResponseBodyIterator(object):
 
     def __iter__(self):
         while True:
-            yield self.next()
+            try:
+                yield self.next()
+            except StopIteration:
+                return
 
     def __bool__(self):
         return hasattr(self, 'items')
@@ -418,7 +421,7 @@ class ResponseBodyIterator(object):
         if chunk:
             return chunk
         else:
-            raise StopIteration()
+            raise StopIteration
 
 
 def _construct_http_client(*args, **kwargs):