The TsigKeyring API for dnspython is inoperable

The TsigKeyring class was intended to implement the dnspython API supporting a callable structure from which the dnspython (https://github.com/rthalley/dnspython/blob/2.2/dns/message.py#L972) issues a call back to the intended TsigKeyring function. Given that the TsigKeyring has no __call__ method it fails the python callable call. As such the TsigKeyring is never called hence the tsig key can't be used to allow a AXFR between the mdns (Designate) and DNS server.