The purpose of doctest's - verify interactive examples.  But if
your users will use custom displayhook all the time (consider, as
examples CAS like SymPy or https://github.com/skirpichev/omg/) - why
you must show them examples with the builtin's displayhook?

After https://bugs.python.org/issue8048, sys.displayhook can't be
customized for doctest.  I think, that decision was wrong and we
should have a better solution.

PS: In fact, right now this issue can be workarrounded if you instead
override sys.__displayhook__ before doctest call.  But I believe
this "solution" has own problems.