stack: add a version pin for subprocess32 ( 3.5.3 )

It will be used on python2.7

from https://docs.python.org/2/library/subprocess.html :
POSIX users (Linux, BSD, etc.) are strongly encouraged to install and
use the much more recent subprocess32 module instead of the version
included with python 2.7. It is a drop in replacement with better
behavior in many situations.

/reviewed-on nexedi/slapos!476