Usually we add plain Python equivalents when they are simple enough that the code equivalent is as understandable as the prose or more (see for example http://docs.python.org/3/library/functions.html#all, or the itertools functions you mentioned).
For this case I think it would help if you presented an equivalent function, e.g.:

def to_bytes(n, length, order):
    if order == 'little':
        return bytes((n >> i*8) & 0xff for i in range(length))
    elif order == 'big':
        return bytes((n >> i*8) & 0xff for i in reversed(range(length)))

or even:

def to_bytes(n, length, order):
    indexes = range(length) if order == 'little' else reversed(range(length))
    return bytes((n >> i*8) & 0xff for i in indexes)

This is also done for http://docs.python.org/3.3/library/stdtypes.html#int.bit_length just above to/from_bytes, so it might be a good addition.
If this is done, the equivalent function can also be added to the test suite, so we can verify that it's indeed equivalent.