switch autoninja shebang to python3

We don't want to use python2 and this doesn't use third party library
too so no need to be vpython which has a small overhead.

Change-Id: Iff924a87f0562069e29195a5693e2260e4a6684e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3400397
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

autoninja.py

-#!/usr/bin/env vpython
+#!/usr/bin/env python3
 # Copyright (c) 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.