docs: def _login(self, username, password):
          """Proceed to login to the Spark Cloud and returns an access token."""
          data = {
              'username': username,
              'password': password,
              'grant_type': 'password'
          }
          r = self.spark_api.oauth.token.POST(auth=('spark', 'spark'), data=data, timeout=self.timeout)
          self._check_error(r)
          return r.json()['access_token']