Given that dictproxy is needed to implement the proper interface for classes I don't think it is an implementation detail, and so I think it ought to be constructable. We have more esoteric types constructable (e.g. function).

I also happen to think that its use to implement classes is a pretty darn good use case, and I am sure there are other use cases.

I don't really care if we make it a builtin but there should be a standard name for it somewhere in the stdlib.