Can't seem to have futurize produce output as advertised

[ewoudvc]

https://github.com/PythonCharmers/python-future/blame/master/docs/futurize.rst#L201
claims that

  for k, v in d.iteritems():
        assert isinstance(v, basestring)

would be converted into with --stage2

  for k, v in iteritems(d):
        assert isinstance(v, (str, bytes))

but I can't seem to trigger this fix.

Looked briefly through lib2to3 and the libfuturize fixers, but didn't find it right away.
Is this a bug in the documentation or is there more than just applying --stage2 to get this?