Portable Python script across Python version

StringIO is obsoleted in Python3, replaced by io.BytesIO or io.StringIO depending on the use.

Differential Revision: https://reviews.llvm.org/D55196

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@350318 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format-diff.py b/clang-format-diff.py
index 54347ce..d6d3510 100755
--- a/clang-format-diff.py
+++ b/clang-format-diff.py
@@ -28,10 +28,11 @@
 import re
 import subprocess
 import sys
-try:
-  from StringIO import StringIO
-except ImportError:
-   from io import StringIO
+
+if sys.version_info.major >= 3:
+    from io import StringIO
+else:
+    from io import BytesIO as StringIO
 
 
 def main():