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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def login(self, username='admin', password='admin'):
 """ Method to authenticate the qBittorrent Client. Declares a class attribute named ``session`` which stores the authenticated session if the login is correct. Else, shows the login error. :param username: Username. :param password: Password. :return: Response to login request to the API. """

        self.session = requests.Session()
        login = self.session.post(self.url+'login',
                                  data={'username': username,
                                        'password': password})
        if login.text == 'Ok.':
            self._is_authenticated = True
        else:
            return login.text