Invoking a subclass's constructor is only an issue when subclass adds 
data members.  In this case, arithmetic methods need to be overridden.  
Note that my patch does not make __add__ and friends invoke subclass' 
constructor, only subclass' tp_alloc.

Existing code already does this in some cases.  For example,

>>> class d(datetime): pass
... 
>>> d.strptime('20080310', '%Y%m%d')
d(2008, 3, 10, 0, 0)
>>> d.now()
d(2008, 3, 10, 20, 27, 6, 303147)

I think date/datetime present a particularly compelling case for 
departing from the general rule.  These classes are minimal by design 
and need to be extended in many applications.