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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def handle_subscription_callback(self, raw, verify_token=model.Subscription.VERIFY_TOKEN_DEFAULT):
 """ Validate callback request and return valid response with challenge. :return: The JSON response expected by Strava to the challenge request. :rtype: Dict[str, str] """

        callback = model.SubscriptionCallback.deserialize(raw)
        callback.validate(verify_token)
        response_raw = {'hub.challenge': callback.hub_challenge}
        return response_raw