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>

ipaserver/dcerpc.py

@@ -55,11 +55,15 @@ from dns import resolver, rdatatype
 from dns.exception import DNSException
 import pysss_nss_idmap
 import pysss
+import six
 from ipaplatform.paths import paths
 
 from ldap.filter import escape_filter_chars
 from time import sleep
 
+if six.PY3:
+    unicode = str
+
 __doc__ = _("""
 Classes to manage trust joins using DCE-RPC calls