add explicit imports in schema declaration

schema.py

-from yams.buildobjs import RichString
+from yams.buildobjs import (MetaUserEntityType, MetaUserRelationType,
+                            ObjectRelation, String, RichString)
+
+from cubicweb.schema import ERQLExpression
 
 class Basket(MetaUserEntityType):
     """a basket contains a set of other entities"""