I haven't tried the example, but at this point I'd be surprised if it failed.  The caching here isn't at level of `__lt__` but at the higher level of (invisible from Python code) a type's tp_richcompare slot.  A heap type - regardless of whether it derives from a C-level or Python-level type - has to be prepared for methods to pop into existence, change, or vanish at (almost) any time.  So its tp_richcompare has to be defensive itself.  For example:

>>> class F(float):
...     pass
>>> a = F(2)
>>> b = F(3)
>>> a < b
True

Is F.tp_richcompare the same as float.tp_richcompare?  We can't tell from Python code, because tp_richcompare isn't exposed.  But, _whatever_ F.tp_richcompare is, it notices when relevant new methods are defined (which float.tp_richcompare emphatically does not); for example, continuing the above:

>>> F.__lt__ = lambda a, b: 0
>>> a < b
0
>>> del F.__lt__
>>> a < b
True

That said, I know nothing about how comparison internals changed for Python 3, so I may just be hallucinating :-)