code: def expires_at(self):
          
          if 'SignatureExpirationTime' in self._signature.subpackets:
              expd = next(iter(self._signature.subpackets['SignatureExpirationTime'])).expires
              return self.created + expd
          return None
docs: A :py:obj:`~datetime.datetime` of when this signature expires, if a signature expiration date is specified.
          Otherwise, ``None``