Flake8: fix errors

- F401 'cubicweb.predicates.is_instance' imported but unused
- F821 undefined name '_'

cubicweb_activitystream/views.py

 from logilab.mtconverter import xml_escape
-from cubicweb.predicates import is_instance, adaptable
+from cubicweb.predicates import adaptable
 from cubicweb.view import EntityView
+from cubicweb import _
 
 
 class AStreamView(EntityView):