PEP 3151 deprecated IOError...

  https://www.python.org/dev/peps/pep-3151/#confusing-set-of-os-related-exceptions

Python 3.3 turned IOError into an OSError alias, so this commit shouldn't
impact our users...

  >>> raise OSError('boom')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  OSError: boom

  >>> raise IOError('boom')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  OSError: boom