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This seems to add a lot of complexity for a few special
cases.  How important are these particular attributes?  Do
you have any benchmark applications that show real
improvement?  It seems like microbenchmarks overstate the
benefit, since we don't know how often these attributes are
looked up by most applications.

It would also be interesting to see how much of the benefit
for non __ names is the result of the PyString_AS_STRING()
macro.  Maybe that's all the change we really need :-).