Made required_draft4 more robust when used with Draft3

Author: William Speirs

jsonschema/_validators.py

@@ -296,6 +296,11 @@ def properties_draft4(validator, properties, instance, schema):
 def required_draft4(validator, required, instance, schema):
     if not validator.is_type(instance, "object"):
         return
+
+    if not isinstance(required, list):
+        yield ValidationError("Found Draft3 style 'required' restriction")
+        return
+
     for property in required:
         if property not in instance:
             yield ValidationError("%r is a required property" % property)