Update cpplint.py's interpreter directive to python3.

Change-Id: Ic6c1ebd049350348cdd0cd4db717bda8c6f3dc34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3540610Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

cpplint.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (c) 2009 Google Inc. All rights reserved.
 #