Fix contextlib.nested to cope with exit methods raising and handling exceptions

Lib/contextlib.py

@@ -127,7 +127,10 @@ def nested(*contexts):
             except:
                 exc = sys.exc_info()
         if exc != (None, None, None):
-            raise
+            # 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], exc[2]
 
 
 @contextmanager