patman: Drop Python 2 StringIO code

We can rely on Python 3 now, so drop the workaround for importing
StringIO.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/func_test.py b/tools/patman/func_test.py
index 76319ff..748e225 100644
--- a/tools/patman/func_test.py
+++ b/tools/patman/func_test.py
@@ -12,10 +12,7 @@
 import tempfile
 import unittest
 
-try:
-    from StringIO import StringIO
-except ImportError:
-    from io import StringIO
+from io import StringIO
 
 import gitutil
 import patchstream