setup: add dependency to subprocess32 on python2

subprocess32 backports the good things from python3's subprocess and is
recommended on https://docs.python.org/2/library/subprocess.html

setup.py

@@ -2,6 +2,7 @@ from setuptools import setup, find_packages
 from shutil import copyfile
 import glob
 import os
+import sys
 
 from slapos.version import version
 name = 'slapos.core'
@@ -27,6 +28,9 @@ try:
 except ImportError:
   additional_install_requires.append('argparse')
 
+if sys.version_info[0] < 3:
+  additional_install_requires.append('subprocess32')
+
 setup(name=name,
       version=version,
       description="SlapOS core.",