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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 is_expired(self):
 """ ``True`` if the signature has an expiration date, and is expired. Otherwise, ``False`` """

        expires_at = self.expires_at
        if expires_at is not None and expires_at != self.created:
            return expires_at < datetime.utcnow()

        return False