docs: def fetch_token(self, code, state):
          """
          Fetch the token, using the verification code. Also, make sure the state
          received in the response matches the one in the request. Returns the
          access_token.
          """
          if self.state != state:
              raise MismatchingStateError()
          self.token = self.oauth.fetch_token(
              '%saccess_token/' % OAUTH_URL, code=code,
              client_secret=self.client_secret)
          return self.token['access_token']