patches/Restricted: fix access check for type

On python2, attributes of new style classes (classes, not instances)
were not properly validated. On python3, attributes of classes were not.

product/ERP5Type/patches/Restricted.py

@@ -124,8 +124,14 @@ class TypeAccessChecker:
     as "a method which returing a method" because we can not know what is the
     type until it is actually called. So the three ways are simulated the
     function returned by this method.
+
+    We don't return a simple function, but a class instance with a __bool__ method
+    to accomodate the two cases where this is called by SecurityManager.validate when
+    checking access on the class (then only the bool is used) or by guarded_getattr
+    when checking access on the instance (the __call__ is used).
     """
-    def factory(inst, name):
+    class _AccessChecker:
+      def __call__(self, inst, name):
         """
         Check function used with ContainerAssertions checked by cAccessControl.
         """
@@ -143,7 +149,12 @@ class TypeAccessChecker:
           # fallback to default security
           aq_acquire(inst, name, aq_validate, getSecurityManager().validate)
         return v
-    return factory
+
+      def __bool__(self):
+        return False
+      __nonzero__ = __bool__ # six.PY2
+
+    return _AccessChecker()
 
   def __bool__(self):
     # If Containers(type(x)) is true, ZopeGuard checks will short circuit,