fixing response iterator in swift3 test

Change-Id: I6e58eed72aa3d1835b1a9ef99014d27a7dfd810e

Author: sasimpson

test/unit/common/middleware/test_swift3.py

@@ -134,8 +134,8 @@ def __call__(self, env, start_response):
                 if 'HTTP_RANGE' in env:
                     resp = Response(body=self.object_body,
                                     conditional_response=True)
-                    iter = resp(env, start_response)
-                    return iter.pop()
+                    resp_iter = iter(resp(env, start_response))
+                    return resp_iter.next()
                 start_response(Response().status,
                                self.response_headers.items())
                 if env['REQUEST_METHOD'] == 'GET':