Migrate to Python 3

Python 2 will not be maintained past April 2020. Fortunately our scripts
require almost no changes to continue working under Python 3.

This patch forces the use of `python3` binary for all `action*` build
targets in GN and updates hashbang directives in our scripts.

Change-Id: Ic34ac547dfe5f85023ddcf4d1a969818f94a72fb
diff --git a/build/parse_strace_open.py b/build/parse_strace_open.py
index 5dd878b..a971220 100755
--- a/build/parse_strace_open.py
+++ b/build/parse_strace_open.py
@@ -1,4 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
+#
 # Copyright 2019 The Hafnium Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");