I dislike the idea of adding a default length to int.to_bytes(). The length changes the meaning of the output:

>>> (1).to_bytes(2, 'big')
b'\x00\x01'
>>> (1).to_bytes(1, 'big')
b'\x01'

If the intent is to "magically cast an integer to a byte strings", having a fixed length of 1 doesn't help:

>>> (1000).to_bytes(1, "big")
OverflowError: int too big to convert

If the intent is to create a bytes string of length 1, I'm not sure that "re-using" this existing API for that is a good idea.