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
1 file changed