code: def handle_subscription_callback(self, raw,
                                       verify_token=model.Subscription.VERIFY_TOKEN_DEFAULT):
          
          callback = model.SubscriptionCallback.deserialize(raw)
          callback.validate(verify_token)
          response_raw = {'hub.challenge': callback.hub_challenge}
          return response_raw
docs: Validate callback request and return valid response with challenge.
  
          :return: The JSON response expected by Strava to the challenge request.
          :rtype: Dict[str, str]