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As to the rationale for having it in the standard library: it's because
of the coupling with the implementation of the type() builtin. If a new
slot is added to type() that the interpreter may access without
consulting __getattribute__, then ProxyMixin can be updated to correctly
delegate that slot. If alternate implementations such as Jython or
IronPython have additional such slots, they can also provide their own
version of ProxyMixin.
If ProxyMixin doesn't become the development responsibility of the same
group that is responsible for the implementation of the type object then
it may as well not exist (since it can't be trusted to be kept up to
date with the development process).