Remove use of six.iteritems()

In Python 3, dict.items() is always an iterator.

tests/test_ext_autosummary.py

@@ -10,7 +10,7 @@
 """
 
 import pytest
-from six import iteritems, StringIO
+from six import StringIO
 
 from sphinx.ext.autosummary import mangle_signature, import_by_name, extract_summary
 from sphinx.testing.util import etree_parse
@@ -140,7 +140,7 @@ def test_get_items_summary(make_app, app_params):
         'C.prop_attr2': 'This is a attribute docstring',
         'C.C2': 'This is a nested inner class docstring',
     }
-    for key, expected in iteritems(expected_values):
+    for key, expected in expected_values.items():
         assert autosummary_items[key][2] == expected, 'Summary for %s was %r -'\
             ' expected %r' % (key, autosummary_items[key], expected)