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/plugins/automember.py

@@ -20,6 +20,7 @@ import uuid
 import time
 
 import ldap as _ldap
+import six
 
 from ipalib import api, errors, Str, StrEnum, DNParam, _, ngettext
 from ipalib.plugable import Registry
@@ -27,6 +28,9 @@ from ipalib.plugins.baseldap import *
 from ipalib.request import context
 from ipapython.dn import DN
 
+if six.PY3:
+    unicode = str
+
 __doc__ = _("""
 Auto Membership Rule.
 """) + _("""