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/aci.py

@@ -119,6 +119,8 @@ targetattr REPLACES the current attributes, it does not add to them.
 """
 from copy import deepcopy
 
+import six
+
 from ipalib import api, crud, errors
 from ipalib import Object
 from ipalib import Flag, Str, StrEnum, DNParam
@@ -130,6 +132,9 @@ from ipalib.plugins.baseldap import gen_pkey_only_option, pkey_to_value
 from ipapython.ipa_log_manager import *
 from ipapython.dn import DN
 
+if six.PY3:
+    unicode = str
+
 register = Registry()
 
 ACI_NAME_PREFIX_SEP = ":"