git-hyper-blame: Make tests run on Python 3.

Bug: 1009809
Change-Id: If3099898a75837d3cee35bf9920680665dd4d322
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1838257
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

tests/git_hyper_blame_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 2016 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.