Marking this as out-of-date.  It seems that the desired functionality has already been added in types.SimpleNamespace:

    # Capabilities of _AttributeHolder
    >>> ah = _AttributeHolder()
    >>> ah
    AttributeHolder()
    >>> ah.raymond='Red'
    >>> ah
    AttributeHolder(raymond='Red')
    >>> ah.raymond
    'Red'
    >>> ah.rachel='blue'
    >>> ah
    AttributeHolder(rachel='blue', raymond='Red')
    >>> ah._get_kwargs()
    [('rachel', 'blue'), ('raymond', 'Red')]

    # Capabilities of SimpleNamespace
    >>> import types
    >>> ah = types.SimpleNamespace()
    >>> ah.raymond='Red'
    >>> ah
    namespace(raymond='Red')
    >>> ah.rachel='blue'
    >>> ah
    namespace(rachel='blue', raymond='Red')
    >>> vars(ah).items()
    dict_items([('raymond', 'Red'), ('rachel', 'blue')])