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>

ipalib/krb_utils.py

@@ -18,10 +18,15 @@
 
 import time
 import re
+
+import six
 import gssapi
 
 from ipalib import errors
 
+if six.PY3:
+    unicode = str
+
 #-------------------------------------------------------------------------------
 
 # Kerberos error codes