python: Better get character ordinals

In Python 2, iterating over a byte-string yields single-byte strings,
and we can pass them to ord() to get the corresponding integer.

In Python 3, iterating over a byte-string directly yields those
integers.

Transforming the byte string into a bytearray gives us a list of the
integers corresponding to each byte in the string, removing the need to
call ord().

This makes the script compatible with both Python 2 and 3.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>