* cause/context cycles should be avoided. Naive traceback printing
could become confused, and I can't think of any accidental way to
provoke it (besides the problem mentioned here.)
* I suspect PyErr_Display handled string exceptions in 2.x, and this is
an artifact of that
* No opinion on PyErr_DisplaySingle
* PyErr_Display is used by PyErr_Print, and it must end up with no
active exception. Additionally, third party code may depend on this
semantic. Maybe PyErr_DisplayEx?
* +1 on standardizing tracebacks