dnspython: Add compatibility shim

`dnspython` 2.0.0 has many changes and several deprecations like:

```
> dns.resolver.resolve() has been added, allowing control of whether
search lists are used. dns.resolver.query() is retained for backwards
compatibility, but deprecated. The default for search list behavior can
be set at in the resolver object with the use_search_by_default
parameter. The default is False.

> dns.resolver.resolve_address() has been added, allowing easy
address-to-name lookups.
```

The new class `DNSResolver`:
- provides the compatibility layer
- defaults the previous behavior (the search list configured in the
  system's resolver configuration is used for relative names)
- defaults lifetime to 15sec (determines the number of seconds
  to spend trying to get an answer to the question)

Fixes: https://pagure.io/freeipa/issue/8383
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipaserver/plugins/host.py

@@ -22,8 +22,6 @@ from __future__ import absolute_import
 
 import logging
 
-import dns.resolver
-
 import six
 
 from ipalib import api, errors, util
@@ -64,7 +62,7 @@ from ipapython.ipautil import (
     CheckedIPAddress,
     TMP_PWD_ENTROPY_BITS
 )
-from ipapython.dnsutil import DNSName
+from ipapython.dnsutil import DNSName, zone_for_name
 from ipapython.ssh import SSHPublicKey
 from ipapython.dn import DN
 from ipapython import kerberos
@@ -826,7 +824,7 @@ class host_del(LDAPDelete):
         if updatedns:
             # Remove A, AAAA, SSHFP and PTR records of the host
             fqdn_dnsname = DNSName(fqdn).make_absolute()
-            zone = DNSName(dns.resolver.zone_for_name(fqdn_dnsname))
+            zone = DNSName(zone_for_name(fqdn_dnsname))
             relative_hostname = fqdn_dnsname.relativize(zone)
 
             # Get all resources for this host