Fix a poorly-translated raise statement in contextlib.

Lib/contextlib.py

@@ -125,7 +125,8 @@ def nested(*managers):
             # Don't rely on sys.exc_info() still containing
             # the right information. Another exception may
             # have been raised and caught by an exit method
-            raise exc[0](exc[1]).with_traceback(exc[2])
+            # exc[1] already has the __traceback__ attribute populated
+            raise exc[1]
 
 
 class closing(object):