dns: Fix support for dnspython 1.1x
    
    `nameservers` was transformed into the property in dnspython 2:
    https://github.com/rthalley/dnspython/commit/bbf0cfd239ffa6deeb67a4787bd292e9a972af74
    
    This causes
    > AttributeError: type object 'Resolver' has no attribute 'nameservers'
    on the previous dnspython 1.1x.
    
    Fixes: https://pagure.io/freeipa/issue/9339
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>