client: convert to python3

See merge request !1

Conversion done by 2to3 and manual fixup afterwards.

Signed-off-by: Rouven Czerwinski <rouven@czerwinskis.de>

offs only contributes to the row address, which is the same regardless
of the selected brightness subframe.

This was long calculations can be done while the client is waiting for
the response packet (which is empty anyway).