[Cython] Bug in DictNode

[Martin C. Martin]

Hi,

Calling get_child_accessors() on a DictNode throws an exception, 
because, because it doesn't define a child_attrs field (and thus 
inherits the value None from class Node), so get_child_attrs() checks 
self.subexprs to see if it's None, but the field doesn't exist.  So, 
what's the right fix?

- Change get_child_attrs() to look at self.saved_subexpr_nodes instead 
of self.subexprs?
- Add "subexprs = None" to ExprNode?
- Add "subexprs = None" to DictNode?
- Something else?

Best,
Martin