[gyb] Python 2 and 3 compatible StringIO import
[gyb] Python 2 and 3 compatible StringIO import

The `StringIO` and `cStringIO` modules are gone in Python 3. The
recommendation is to import the `io` module on Python 3. Therefore, this
patch first attempts to import the Python 2 module, `cStringIO`, and if
that fails then attempts to import the Python 3 module, `io`.

**NOTE**: There are still other Python 3.x fixes necessary to make `gyb`
run on a Python 3.x interpreter. This is just one small incremental
patch on the way there.

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