Py3: Replace six.string_types with str

In Python 3, six.string_types is just an alias for str.

See: https://pagure.io/freeipa/issue/7715
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipapython/dnsutil.py

@@ -49,7 +49,7 @@ class DNSName(dns.name.Name):
 
     def __init__(self, labels, origin=None):
         try:
-            if isinstance(labels, six.string_types):
+            if isinstance(labels, str):
                 #pylint: disable=E1101
                 labels = dns.name.from_text(unicode(labels), origin).labels
             elif isinstance(labels, dns.name.Name):