Handle StopIteration for Py3.7 PEP 0479

Replace raise of StopIteration with return.

PEP 0479, https://www.python.org/dev/peps/pep-0479/, makes the
following change: "when StopIteration is raised inside a generator,
it is replaced it with RuntimeError".
And states: "If raise StopIteration occurs directly in a generator,
simply replace it with return."

Change-Id: Ia7d6af7165077aa93c76185c28c355c06a202088
Closes-Bug: #1781627

glance/tests/unit/api/test_common.py

@@ -34,7 +34,7 @@ class SimpleIterator(object):
             yield chunk
             chunk = read_chunk()
         else:
-            raise StopIteration()
+            return
 
 
 class TestSizeCheckedIter(testtools.TestCase):

glance/tests/unit/common/test_utils.py

@@ -87,7 +87,7 @@ class TestUtils(test_utils.BaseTestCase):
             yield chunk
             iteration += 1
             if iteration >= max_iterations:
-                raise StopIteration()
+                return
 
     def _test_reader_chunked(self, chunk_size, read_size, max_iterations=5):
         generator = self._create_generator(chunk_size, max_iterations)