Partition the bytearray into three parts using the given separator.
This will search for the separator sep in the bytearray, starting at
the end. 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 two
empty bytearray objects and the copy of the original bytearray
object.