Ignore stderr when checking for Python version

https://review.openstack.org/504171 prevented Python version detection
from failing when python3 is not installed. However, "which python3"
returns a message in stderr when python3 is not there, and this output
can make diskimage-builder get confused when parsing
source-repository-images.

Change-Id: Idb649dc341ede73c39954b0432ef3cacf379ed37
1 file changed