hangs for a long time in query_ns when deleting a host

[ThomasWaldmann]

This is traceback from ctrl-c (after waiting for ~30 minutes):

    user.delete()
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/django/db/models/base.py", line 974, in delete
    return collector.delete()
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/django/db/models/deletion.py", line 279, in delete
    sender=model, instance=obj, using=self.using
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/django/dispatch/dispatcher.py", line 193, in send
    for receiver in self._live_receivers(sender)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/django/dispatch/dispatcher.py", line 193, in <listcomp>
    for receiver in self._live_receivers(sender)
  File "/srv/nsupdate.info/repo/src/nsupdate/main/models.py", line 348, in pre_delete_host
    dnstools.delete(obj.get_fqdn())
  File "/srv/nsupdate.info/repo/src/nsupdate/main/dnstools.py", line 180, in delete
    query_ns(fqdn, rdtype)
  File "/srv/nsupdate.info/repo/src/nsupdate/main/dnstools.py", line 260, in query_ns
    answer = resolver.query(str(fqdn), rdtype)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/resolver.py", line 913, in query
    source_port=source_port)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/query.py", line 325, in udp
    q.keyring, q.mac, ignore_trailing)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/query.py", line 258, in receive_udp
    _wait_for_readable(sock, expiration)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/query.py", line 156, in _wait_for_readable
    _wait_for(s, True, False, True, expiration)
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/query.py", line 131, in _wait_for
    if not _polling_backend(fd, readable, writable, error, timeout):
  File "/srv/nsupdate.info/env/lib/python3.7/site-packages/dns/query.py", line 90, in _poll_for
    event_list = pollable.poll(long(timeout * 1000))
KeyboardInterrupt