diff -r 6b3523f81f42 -r 26744ad37953 cubicweb/web/views/uicfg.py
--- a/cubicweb/web/views/uicfg.py	Fri Apr 05 17:21:14 2019 +0200
+++ b/cubicweb/web/views/uicfg.py	Fri Apr 05 17:58:19 2019 +0200
@@ -56,8 +56,6 @@
 
 from itertools import repeat
 
-from six import string_types
-
 from cubicweb import neg_role
 from cubicweb.rtags import (RelationTags, RelationTagsBool, RelationTagsSet,
                             RelationTagsDict, NoTargetRelationTagsDict,
@@ -692,7 +690,7 @@
                 self.tag_relation((sschema, rschema, oschema, role), True)
 
     def _tag_etype_attr(self, etype, attr, desttype='*', *args, **kwargs):
-        if isinstance(attr, string_types):
+        if isinstance(attr, str):
             attr, role = attr, 'subject'
         else:
             attr, role = attr