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/service.py

@@ -22,6 +22,7 @@
 import base64
 import os
 
+import six
 
 from ipalib import api, errors, util
 from ipalib import Str, Flag, Bytes, StrEnum, Bool
@@ -34,6 +35,9 @@ import nss.nss as nss
 from nss.error import NSPRError
 from ipapython.ipautil import file_exists
 
+if six.PY3:
+    unicode = str
+
 __doc__ = _("""
 Services