I discovered the hard way today that this won't work:

>>> import datetime
>>> d = datetime.timedelta(1)
>>> d / 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'datetime.timedelta' and 'int'

The error message is misleading, because in fact timedelta
objects *do* support division by ints, just not with the
/ operator:

>>> d // 2
datetime.timedelta(0, 43200)

Is there some way the error message can be improved,
perhaps by identifying the denominator as effectively
a float?

If you set nb_true_div on timedelta objects to delta_divide (just like
nb_floor_div) you won't have this problem as the division will just
work.  Otherwise there is no other good way as if the divisor doesn't
work you need to return NotImplemented, which then tries the right-hand
object which fails, and 'object' returns the TypeError.  Best you could
do is a warning or something.  

But it might make sense to only set it on nb_true_div as the division
does not round anything.

The current behaviour is consistent with the integer divison:
>>> 21 // 10
2
>>> timedelta(microseconds=20) // 10
datetime.timedelta(0, 0, 2)

Whereas int/int gives float:
>>> 21 / 10
2.1000000000000001
>> timedelta(microseconds=20) / 1
...
TypeError: unsupported operand type(s) for /: ...

Now in the real world, it's hard to understand that the operator // 
should be used instead of /. So timedelta()/int might be an alias to 
timedelta()//int.