Allow for immutable subclasses of YAMLObject

[toidinamai]

YAMLObject are not directly instantiated so they don't need their own instance attribute (i.e. an own dict). Because of that they should define an empty slots and let subclasses decide whether they want arbitrary instance attributes or not:

--- lib/yaml/__init__.py        (revision 251)
+++ lib/yaml/__init__.py        (working copy)
@@ -272,6 +272,9 @@
     yaml_tag = None
     yaml_flow_style = None
 
+    # no direct instantiation, so allow for immutable subclasses
+    __slots__ = ()
+
     def from_yaml(cls, loader, node):
         """
         Convert a representation node to a Python object.

The same is basically also true for YAMLObjectMetaclass but I doubt there is much use for this in practice.

[xi]

Makes sense to me. Applied in [252]. Thanks for the patch.