Surely remote proxies fall under what would be expected for a "proxy
mixin"?  If it's in the stdlib it should be a canonical implementation,
NOT a reference implementation.

At the moment I can think up 3 use cases:
* weakref proxies
* lazy load proxy
* distributed object

The first two could be done if _deref were made overridable.  The third
needs to turn everything into a message, which would could either do
directly, or we could do by turning everything into normal method
lookups which then get handled through __getattribute__.