Do not raise StopIteration

The current code will not work in Python3.7 because of PEP479. This
commit fixes it so that it works with both 3.6 and 3.7.

[1] https://www.python.org/dev/peps/pep-0479/

Closes-bug: #1815335
Change-Id: I6adf23e0569db8655806a07d53c5000f5fd0918f

glance_store/_drivers/rbd.py

@@ -241,7 +241,7 @@ class ImageIterator(object):
                             data = image.read(size - bytes_left, length)
                             bytes_left -= len(data)
                             yield data
-                        raise StopIteration()
+                        return
         except rbd.ImageNotFound:
             raise exceptions.NotFound(
                 _('RBD image %s does not exist') % self.name)

glance_store/_drivers/sheepdog.py

@@ -256,7 +256,7 @@ class ImageIterator(object):
             data = image.read(total - left, length)
             left -= len(data)
             yield data
-        raise StopIteration()
+        return
 
 
 class Store(glance_store.driver.Store):