'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.'

def login(self, username, password):

    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
