Just use a bytearray (Python 2.6 and later) which represents a mutable sequence of bytes
>>> key = bytearray([0x13, 0x00, 0x00, 0x00, 0x08, 0x00])
>>> key
bytearray(b'\x13\x00\x00\x00\x08\x00')
Indexing get and sets the individual bytes
>>> key[0]
19
>>> key[1]=0xff
>>> key
bytearray(b'\x13\xff\x00\x00\x08\x00')
and if you need it as a str (or bytes in Python 3), it's as simple as
>>> bytes(key)
'\x13\xff\x00\x00\x08\x00'