It is implemented as a doctest because everything else in that test module is implemented as a doctest.  And clearly this reveals a bug of some sort.  Perhaps the dummy trick needs to be done by doctest itself?  (I will need to read your post over more carefully and understand the issue better.)

As for the windows failures, my apologies.  I knew I would need to check windows after the commit when I wrote the code, but I forgot to actually do it.