[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.

utils/gyb.py

@@ -5,7 +5,10 @@
 from __future__ import print_function
 
 import re
-from cStringIO import StringIO
+try:
+    from cStringIO import StringIO
+except ImportError:
+    from io import StringIO
 import tokenize
 import textwrap
 from bisect import bisect