Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid using six.iteritems to achieve iterators.
We can use dict.items instead, as it will return iterators in PY3 as well.
And dict.items/keys will more readable.
2.In py2, the performance about list should be negligible, see the link [2].

[1] https://wiki.openstack.org/wiki/Python3
[2] http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: If76115f706e9e100a1780471af0f9c5d92d3bc9f

oslo_db/sqlalchemy/models.py

@@ -92,7 +92,7 @@ class ModelBase(six.Iterator):
 
     def update(self, values):
         """Make the model object behave like a dict."""
-        for k, v in six.iteritems(values):
+        for k, v in values.items():
             setattr(self, k, v)
 
     def _as_dict(self):
@@ -101,14 +101,14 @@ class ModelBase(six.Iterator):
         Includes attributes from joins.
         """
         local = dict((key, value) for key, value in self)
-        joined = dict([(k, v) for k, v in six.iteritems(self.__dict__)
+        joined = dict([(k, v) for k, v in self.__dict__.items()
                       if not k[0] == '_'])
         local.update(joined)
         return local
 
     def iteritems(self):
         """Make the model object behave like a dict."""
-        return six.iteritems(self._as_dict())
+        return self._as_dict().items()
 
     def items(self):
         """Make the model object behave like a dict."""

oslo_db/tests/sqlalchemy/test_utils.py

@@ -18,7 +18,6 @@ import mock
 from oslo_utils import uuidutils
 from oslotest import base as test_base
 from oslotest import moxstubout
-import six
 from six.moves.urllib import parse
 import sqlalchemy
 from sqlalchemy.dialects import mysql
@@ -614,7 +613,7 @@ class TestMigrationUtils(db_test_base.DbTestCase):
         }
 
         index_instances = [Index(name, *columns)
-                           for name, columns in six.iteritems(indexes)]
+                           for name, columns in indexes.items()]
 
         table = Table(table_name, self.meta,
                       Column('id', Integer, primary_key=True),