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/trust.py

@@ -18,6 +18,8 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import six
+
 from ipalib.plugable import Registry
 from ipalib.plugins.baseldap import *
 from ipalib.plugins.dns import dns_container_exists
@@ -29,6 +31,9 @@ from ipalib import errors
 from ldap import SCOPE_SUBTREE
 from time import sleep
 
+if six.PY3:
+    unicode = str
+
 try:
     import pysss_murmur #pylint: disable=F0401
     _murmur_installed = True