python: Use list comprehension in string_to_byte_array()

python: Use list comprehension in string_to_byte_array()

Samba is now a mature user of Python and can cope with a
list comprehension from time to time.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>