Updates to the with-statement:

- 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.

Python/errors.c

@@ -24,6 +24,7 @@ PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
 
 	if (traceback != NULL && !PyTraceBack_Check(traceback)) {
 		/* XXX Should never happen -- fatal error instead? */
+		/* Well, it could be None. */
 		Py_DECREF(traceback);
 		traceback = NULL;
 	}