docs: def compute_tls13_resumption_secret(self):
          """
          self.handshake_messages should be ClientHello...ClientFinished.
          """
          if self.connection_end == "server":
              hkdf = self.prcs.hkdf
          elif self.connection_end == "client":
              hkdf = self.pwcs.hkdf
          rs = hkdf.derive_secret(self.tls13_master_secret,
                                  b"resumption master secret",
                                  b"".join(self.handshake_messages))
          self.tls13_derived_secrets["resumption_secret"] = rs