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/test_util.py b/tools/patman/test_util.py
index 4338dbc..4d0085e 100644
--- a/tools/patman/test_util.py
+++ b/tools/patman/test_util.py
@@ -10,10 +10,7 @@
 
 import command
 
-try:
-  from StringIO import StringIO
-except ImportError:
-  from io import StringIO
+from io import StringIO
 
 PYTHON = 'python%d' % sys.version_info[0]