Check that ``username`` and ``password`` have been set, and raise an
        exception if not.

def check_credentials(self):
        """
        Check that ``username`` and ``password`` have been set, and raise an
        exception if not.
        """
        if self.username is None or self.password is None:
            raise DistlibException('username and password must be set')
        pm = HTTPPasswordMgr()
        _, netloc, _, _, _, _ = urlparse(self.url)
        pm.add_password(self.realm, netloc, self.username, self.password)
        self.password_handler = HTTPBasicAuthHandler(pm)