Replace dict.items() with list(dict.items())

Python 3 dict.items() returns an iterator instead
of list, so typecasting is required where necessary.

Partially-Implements: blueprint designate-py3

Change-Id: Ief4c48639fc7b8f562527640b641cf201826dd4a

designate/schema/__init__.py

@@ -79,7 +79,7 @@ class Schema(object):
 
         filtered = {}
 
-        for name, subschema in properties.items():
+        for name, subschema in list(properties.items()):
             if 'type' in subschema and subschema['type'] == 'array':
                 subinstance = instance.get(name, None)
                 filtered[name] = self._filter_array(subinstance, subschema)