Another frozendict usage example: freeze the dict of a new type if it contains __final__ in its namespace.

Example:

>>> class Classic:
...     pass
... 
>>> Classic.attr=1
>>> class FinalizedType:
...     __final__=True
... 
>>> FinalizedType.attr=1
(...)
TypeError: 'frozendict' object does not support item assignment

This patch is not part of the PEP 416 and is just a proof-of-concept.