[fix, calendar] Ensure that the buffer is a string

'unicode' is not defined in python3

--HG--
branch : 3.26

cubicweb/web/views/calendar.py

@@ -23,6 +23,7 @@ from cubicweb import _
 import copy
 from datetime import timedelta
 
+import six
 from logilab.mtconverter import xml_escape
 from logilab.common.date import todatetime
 
@@ -100,9 +101,7 @@ try:
                 if ical_task.stop:
                     elt.add(stop_kw).value = ical_task.stop
 
-            buff = ical.serialize()
-            if not isinstance(buff, unicode):
-                buff = unicode(buff, self._cw.encoding)
+            buff = six.ensure_str(ical.serialize(), encoding=self._cw.encoding)
             self.w(buff)
 
 except ImportError: