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.