No module named 'cStringIO' under Python 3

[mprogram]

apptools for Python 3 may not have module named 'cStringIO', please have a look at: https://stackoverflow.com/q/28200366

Related issue: enthought/mayavi/issues/797

[mdickinson]

We've recently moved the codebase from using 2to3 to using six, so we should be using either six.BytesIO or six.StringIO as appropriate.