dnsutil: Rename __nonzero__ to __bool__

In Python 3, this special method got renamed. Set both to the same
function to keep compatibility.

https://fedorahosted.org/freeipa/ticket/5623

Reviewed-By: Jan Cholasta <jcholast@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>

ipapython/dnsutil.py

@@ -49,11 +49,13 @@ class DNSName(dns.name.Name):
             # instead of a dns.exception
             raise dns.exception.SyntaxError(e)
 
-    def __nonzero__(self):
+    def __bool__(self):
         #dns.name.from_text('@') is represented like empty tuple
         #we need to acting '@' as nonzero value
         return True
 
+    __nonzero__ = __bool__  # for Python 2
+
     def __copy__(self):
         return DNSName(self.labels)