Fix dns import

twisted.protocols.dns was moved to twisted.names.dns long ago in
Twisted 2.0. There used to be a compatibility layer in
twisted.protocols.dns, but that was removed in Twisted 9.0.

https://bugs.launchpad.net/ubuntu/+source/ldaptor/+bug/881404

bin/ldaptor-find-server

@@ -4,8 +4,7 @@ import sys
 from ldaptor.protocols.ldap.distinguishedname import DistinguishedName
 from twisted.internet.defer import DeferredList
 from twisted.internet import reactor
-from twisted.protocols import dns
-from twisted.names import client
+from twisted.names import client, dns
 
 def printAnswer(answers, dn):
     for rrlist in answers: