self.handshake_messages should be ClientHello...ClientFinished.

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