Futurize maps StringIO.StringIO to io.StringIO

[Skip Montanaro]

I had some deja vu recently, cuz of this old thread:

https://mail.python.org/pipermail/python-list/2013-May/648245.html

Today the context is different, but the problem remains the same. One
of the futurize refactoring tools converts usage of StringIO.StringIO
to io.StringIO, which, given that the latter accepts just about
anything you through at it, is a bit surprising. Perhaps there is no
true corresponding class in the io module?

Skip