Could you, for posterity, explain what is happening here? (I wrote this so long ago I cannot remember how the metaclass stuff worked).

I haven't completely wrapped my head around metaclasses but from what I understand the fix works because the return statement return type.__new__(...) in metaCookie's __new__(...) function returns an instance of metaCookie.
This is why the __init__ of metaCookie is called. Adding in the expires property at init stage instead doesn't cause any issues.

However, I'm not sure what changed in the newer versions of python that actually trigger ValueError: 'expires' in __slots__ conflicts with class variable...