Use cStringIO if available.

This attempts to import cStringIO then will fall back on the
six.StringIO import if cStringIO can't be found. This means on python2
we should get cStringIO if available otherwise we get the right StringIO
on both python2 and python3.

Change-Id: I046eb72f6b6ed4801ea43900d2516918fdefbe95

tests/base.py

@@ -28,7 +28,10 @@ import re
 import select
 import shutil
 from six.moves import reload_module
-from six import StringIO
+try:
+    from cStringIO import StringIO
+except Exception:
+    from six import StringIO
 import socket
 import string
 import subprocess