Partition the bytearray into three parts using the given separator.
This will search for the separator sep in the bytearray. If the
separator is found, returns a 3-tuple containing the part before the
separator, the separator itself, and the part after it as new
bytearray objects.
If the separator is not found, returns a 3-tuple containing the copy
of the original bytearray object and two empty bytearray objects.