make module work with Python 3

For the discid module encoding and decoding
is necessary, because the Library gives byte strings.
Python 2 doesn't care, but Python 3 makes a difference.

We only encode/decode to/from ASCII.
Device names and disc ids are ASCII.
document that this works with Python 2 and 3

The minimum version is set to Python 2.6.

print(string) works for Python 2 and 3 unchanged,
because (string) is just a 1-tuple for Python 2.

However this is coincidence and we explicitely want to
document that we cater for Python 2 and 3 in the examples.