Replace six.iteritems with dict.items(Part-1)

according to https://wiki.openstack.org/wiki/Python3,
now we should avoid using six.iteritems and replace
it with dict.items.

Change-Id: I8753e80b34c0f86cf70aebc3bcbd3392ee933f62
Partial-Bug: #1680761
diff --git a/neutron/tests/tempest/api/test_routers.py b/neutron/tests/tempest/api/test_routers.py
index d6beb77..7a1e458 100644
--- a/neutron/tests/tempest/api/test_routers.py
+++ b/neutron/tests/tempest/api/test_routers.py
@@ -14,7 +14,6 @@
 #    under the License.
 
 import netaddr
-import six
 from tempest.lib.common.utils import data_utils
 from tempest.lib import decorators
 from tempest import test
@@ -91,7 +90,7 @@
             self.assertIsNone(actual_ext_gw_info)
             return
         # Verify only keys passed in exp_ext_gw_info
-        for k, v in six.iteritems(exp_ext_gw_info):
+        for k, v in exp_ext_gw_info.items():
             self.assertEqual(v, actual_ext_gw_info[k])
 
     def _verify_gateway_port(self, router_id):