setup.py: fix compatibility for python 2.6

For python3 Popen.communicate returns a bytes type which has no read
function. Therefore for python3 compatibility a decode call has been
introduced. For 2.6 <= python version < 3 (versions prior 2.6 has not
been tested) the bytes type points to str type. Use decode calls just
for the case that str type and bytes type are not equal.

Change-Id: Ia76fc8132e154b9401bb0d09ee311f3e6b3fc374
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/60
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>