package/python-posix-ipc: enable also for python3

The python-posix-ipc package can be used both with python2 and python3.

Signed-off-by: Daniel Fahlgren <daniel@fahlgren.se>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>