Alias "unicode" to "str" under Python 3

The six way of doing this is to replace all occurences of "unicode"
with "six.text_type". However, "unicode" is non-ambiguous and
(arguably) easier to read. Also, using it makes the patches smaller,
which should help with backporting.

Reviewed-By: Petr Viktorin <pviktori@redhat.com>

ipapython/dnsutil.py

@@ -23,6 +23,9 @@ import copy
 
 import six
 
+if six.PY3:
+    unicode = str
+
 
 @six.python_2_unicode_compatible
 class DNSName(dns.name.Name):