Use python3 for all of Dawn's python scripts

Bug: dawn:389
Change-Id: I5034ab2791f0cca0b7690eae5f4d7f4e6efc4bd9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19863
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/generator/extract_json.py b/generator/extract_json.py
index 40312f9..4afefae 100644
--- a/generator/extract_json.py
+++ b/generator/extract_json.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # Copyright 2018 The Dawn Authors
 #
 # Licensed under the Apache License, Version 2.0 (the "License");