tools/python: fix Python3.4 TypeError in format string

tools/python: fix Python3.4 TypeError in format string

Using the first element of a tuple for a format specifier fails with
python3.4 as included in SLE12:
    b = b"string/%x" % (i, )
TypeError: unsupported operand type(s) for %: 'bytes' and 'tuple'

It happens to work with python 2.7 and 3.6.
To support older Py3, format as strings and explicitly encode as ASCII.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>