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/scripts/perf_test_runner.py b/scripts/perf_test_runner.py
index 546bebd..c4cc55c 100755
--- a/scripts/perf_test_runner.py
+++ b/scripts/perf_test_runner.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python3
 #
 # Copyright 2019 The Dawn Authors
 #