backport stable

schema.py

-from yams.buildobjs import EntityType, String, SubjectRelation
+from yams.buildobjs import EntityType, String, RichString, SubjectRelation
 from cubicweb.schema import WorkflowableEntityType, ERQLExpression
@@ -2,9 +2,4 @@
 from cubicweb.schema import WorkflowableEntityType, ERQLExpression
-try:
-    from yams.buildobjs import RichString
-except ImportError:
-    from cubicweb.schema import RichString
-
 
 class Blog(EntityType):
     title = String(maxsize=50, required=True)