From the HTML source you can tell the keys you are looking for are 'dob_m', 'dob_d' and 'dob_y'. You should be able to login to a birthday locked account using this:
In the example above the username is user, the password is pass and the birthday is January 2nd, 1990.Code:self.post('/login.phtml', {'username': 'user', 'password': 'pass', 'destination': "/index.phtml", 'dob_m': '01', 'dob_y': '1990', 'dob_d': '02'}, readable = False)