docs: def encap(self, pkt):
          """encapsulate a frame using this Secure Association"""
          if pkt.name != Ether().name:
              raise TypeError('cannot encapsulate packet in MACsec, must be Ethernet')  # noqa: E501
          hdr = copy.deepcopy(pkt)
          payload = hdr.payload
          del hdr.payload
          tag = MACsec(sci=self.sci, an=self.an,
                       SC=self.send_sci,
                       E=self.e_bit(), C=self.c_bit(),
                       shortlen=MACsecSA.shortlen(pkt),
                       pn=(self.pn & 0xFFFFFFFF), type=pkt.type)
          hdr.type = ETH_P_MACSEC
          return hdr / tag / payload