python2 --> python3

Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).

tools/umm.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 #
 # Copyright 2017, Data61