bug fix in ast dump

@@ -116,7 +116,7 @@ class ASTPrinterVisitor(Visitor):
         for key in dir(node):
             if key[0] == '_':
                 continue
-            val = node.__dict__[key]
+            val = getattr(node, key, None)
             if (isinstance(val, str) or
                 isinstance(val, int)):
                 self.p("  %s: %s" % (key, str(val)))