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/sudorule.py

@@ -18,6 +18,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import netaddr
+import six
 
 from ipalib import api, errors
 from ipalib import Str, StrEnum, Bool, Int
@@ -35,6 +36,9 @@ from ipalib import _, ngettext
 from ipalib.util import validate_hostmask
 from ipapython.dn import DN
 
+if six.PY3:
+    unicode = str
+
 __doc__ = _("""
 Sudo Rules
 """) + _("""