Remove the ability to slice/index on exceptions per PEP 352.
diff --git a/Lib/test/test_file.py b/Lib/test/test_file.py
index 7eb052b..a57ab43 100644
--- a/Lib/test/test_file.py
+++ b/Lib/test/test_file.py
@@ -156,7 +156,7 @@
         try:
             f = open(TESTFN, bad_mode)
         except ValueError as msg:
-            if msg[0] != 0:
+            if msg.message != 0:
                 s = str(msg)
                 if s.find(TESTFN) != -1 or s.find(bad_mode) == -1:
                     self.fail("bad error message for invalid mode: %s" % s)