Subprocess management: use a decorator

Use a decorator instead of a mixin to handle subprocess spawning and
monitoring

bpackage/package.py

@@ -1,8 +1,8 @@
 #!/usr/bin/env python3
 
 # HACK: seems 'requests' module bundled with blender isn't bundled with 'idna' module. So force system python for now
-# import sys
-# sys.path.insert(0, '/usr/lib/python3.6/site-packages')
+import sys
+sys.path.insert(0, '/usr/lib/python3.6/site-packages')
 
 import logging