An example where getting correct rounding is awkward: the following rounds up, with or without the patch:

>>> datetime.timedelta(seconds=0.6112295)
datetime.timedelta(0, 0, 611230)

But in this case, the result should actually be rounded down (with either rounding mode), since the exact value of float('0.6112295') is a little less than 0.6112295:

>>> print(Decimal(0.6112295))
0.61122949999999998116351207499974407255649566650390625

The problem is that leftover_us ends up being exactly 0.5 in this case.  Again, this could be worked around if we really care, but I'm not convinced it's worth it.