python3: add new type of authorized node in condition validator

src/authentic2/utils/evaluate.py

@@ -149,6 +149,13 @@ class ConditionValidator(BaseExpressionValidator):
         ast.Compare,
     ]
 
+    def __init__(self, authorized_nodes=None, forbidden_nodes=None):
+        super(ConditionValidator, self).__init__(
+                authorized_nodes=authorized_nodes,
+                forbidden_nodes=forbidden_nodes)
+        if six.PY3:
+            self.authorized_nodes.append(ast.NameConstant)
+
     def check_Name(self, node):
         if node.id.startswith('_'):
             raise ExpressionError(_('name must not start with a _'), code='invalid-variable', node=node)