Switch all uses of StringIO to use it from six

As part of enabling python3 support in tempest we need to be able to
handle StringIO usage in both python2 and python3. Six provides a
compat layer for doing this, so this commit moves all uses of StringIO
and cStringIO to get it through six.

Change-Id: Ie6ac86b3b5ed2c307dc2cc41386da9e5ba1ee23e
diff --git a/tempest/tests/test_wrappers.py b/tempest/tests/test_wrappers.py
index ae7860d..a4ef699 100644
--- a/tempest/tests/test_wrappers.py
+++ b/tempest/tests/test_wrappers.py
@@ -14,10 +14,11 @@
 
 import os
 import shutil
-import StringIO
 import subprocess
 import tempfile
 
+import six
+
 from tempest.tests import base
 
 DEVNULL = open(os.devnull, 'wb')
@@ -50,8 +51,8 @@
         shutil.copy('tools/pretty_tox_serial.sh',
                     os.path.join(self.directory, 'pretty_tox_serial.sh'))
 
-        self.stdout = StringIO.StringIO()
-        self.stderr = StringIO.StringIO()
+        self.stdout = six.StringIO()
+        self.stderr = six.StringIO()
         # Change directory, run wrapper and check result
         self.addCleanup(os.chdir, os.path.abspath(os.curdir))
         os.chdir(self.directory)