Eric,

Could you share details of your use-case?  My experience with subclassing from basic python types including date/time has been mostly negative.  The problem is that when I subclass, I want to inherit the rich set of operations such as +, -, *, etc., and add a few methods of my own.  After that, I want to always use instances of my subclass instead of the stdlib one.  This does not work because adding instances of my subclass returns an instance of the superclass unless I override __add__ explicitly. (See   #2267.)  This kills all benefits of subclassing as compared to containment.

These days I try to stay away from subclassing date/time, int/float, or anything like that and thus have little incentive to resolve this issue.  And it does not look like we have a workable solution.