Anyway, this is the changes to the with-statement
so that __exit__ must return a true value in order
for a pending exception to be ignored.
The PEP (343) is already updated.

exception, it should not be propagated up.  With unit tests.

- New semantics for __exit__() -- it must re-raise the exception
  if type is not None; the with-statement itself doesn't do this.
  (See the updated PEP for motivation.)

- Added context managers to:
  - file
  - thread.LockType
  - threading.{Lock,RLock,Condition,Semaphore,BoundedSemaphore}
  - decimal.Context

- Added contextlib.py, which defines @contextmanager, nested(), closing().

- Unit tests all around; bot no docs yet.