That's a good idea, Raymond.

>>> [x.__hash__ is None for x in (list, dict, set, frozenset)]
[True, True, True, False]

I don't think this change would cause any backward compatibility issues, except it would now allow a default of something bad like:

>>> class BadList(list):
...   def __hash__(self): return 0
...
>>> isinstance(BadList(), list), BadList.__hash__ is None
(True, False)

I can't say I care too much about now allowing things that didn't used to be allowed, especially if they're poorly designed like BadList.

I'll put together a PR.