tools/pydfu.py: Fix regression so tool runs under Python 2 again.

Under python3 (tested with 3.6.7) bytes with a list of integers as an
argument returns a different result than under python 2.7 (tested with
2.7.15rc1) which causes pydfu.py to fail when run under 2.7.  Changing
bytes to bytearray makes pydfu work properly under both Python 2.7 and
Python 3.6.