update-payload-extractor: Switch to env python

* update_payload now supports both py2 and py3

Change-Id: I9804adda6fd1fcebac75e993702390c0208ad0ef

update-payload-extractor/extract.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 
 import argparse
 import errno