patman: Support use of stringIO in Python 3

With Python 3 this class has moved. Update the code to handle both cases.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/func_test.py

@@ -12,15 +12,20 @@ import sys
 import tempfile
 import unittest
 
+try:
+    from StringIO import StringIO
+except ImportError:
+    from io import StringIO
+
 import gitutil
 import patchstream
 import settings
+import tools
 
 
 @contextlib.contextmanager
 def capture():
     import sys
-    from cStringIO import StringIO
     oldout,olderr = sys.stdout, sys.stderr
     try:
         out=[StringIO(), StringIO()]