[hooks] use cubicweb.predicates instead of cubicweb.selectors

Renamed in 3.15

hooks.py

@@ -8,7 +8,7 @@ from datetime import datetime, timedelta
 from logilab.common.decorators import monkeypatch
 from yams import ValidationError
 
-from cubicweb.selectors import is_instance
+from cubicweb.predicates import is_instance
 from cubicweb.crypto import encrypt
 from cubicweb.server import hook
 from cubicweb.server.repository import Repository