I am probably completely missing the point, but what I retained of the original BDD paper was that BDD was just TDD repackaged to emphasize some best practices.
If my interpretation is correct, you can get a BDD framework just by renaming methods around in any xUnit implementation. So just go ahead and use the standard library's unittest.
EDIT: A quick google turned up a Behaviour module in the Cheese Shop. Further searching for BDD there did not find anything else.