get rid of six

mongoengine/context_managers.py

@@ -1,7 +1,6 @@
 from contextlib import contextmanager
 
 from pymongo.write_concern import WriteConcern
-from six import iteritems
 
 from mongoengine.common import _import_class
 from mongoengine.connection import DEFAULT_CONNECTION_NAME, get_db
@@ -123,7 +122,7 @@ class no_dereference(object):
 
         self.deref_fields = [
             k
-            for k, v in iteritems(self.cls._fields)
+            for k, v in self.cls._fields.items()
             if isinstance(v, (ReferenceField, GenericReferenceField, ComplexBaseField))
         ]