Fix type mismatch trying to difference two lists.

Author: Mike Orr

user_sync/connector/directory_ldap.py

@@ -261,7 +261,7 @@ def iter_users(self, users_filter, extended_attributes):
         user_attribute_names.extend(self.user_username_formatter.get_attribute_names())
         user_attribute_names.extend(self.user_domain_formatter.get_attribute_names())
 
-        extended_attributes -= user_attribute_names
+        extended_attributes = list(set(extended_attributes) - set(user_attribute_names))
         user_attribute_names.extend(extended_attributes)
 
         result_iter = self.iter_search_result(base_dn, ldap.SCOPE_SUBTREE, users_filter, user_attribute_names)