There is another inconsistency that this patch does not seem to cure.  With patch applied and D and DT defined as in OP,

>>> D(1900,1,1) > DT(1900,1,1)
True

but

>>> DT(1900,1,1) < D(1900,1,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare DT to D

and

>>> date(1900,1,1) < datetime(1900,1,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare datetime.datetime to datetime.date

Note that without the patch,

>>> D(1900,1,1) > DT(1900,1,1)
False

but both behaviors seem to be wrong.