re.escape os.sep so that \ is interpreted properly in the regex.

diff --cc Lib/lib2to3/tests/test_main.py
index 225c15de1889c43e078c8f23c01049f1eb8befa9,86cb438e358e763b255e5a58d87ea123a979ffd7..a498c5a0d1dbc1bfe44eb51b98fc33f4ce57218e
--- 1/Lib/lib2to3/tests/test_main.py
--- 2/Lib/lib2to3/tests/test_main.py
+++ b/Lib/lib2to3/tests/test_main.py
@@@ -3,6 -3,6 +3,7 @@@ import codec
  import io
  import logging
  import os
++import re
  import shutil
  import sys
  import tempfile