depot_tools: Run git_cache_test and git_dates_test on Python 3.

Bug: 1009809
Change-Id: I430b5a78ee0f63388d7bed5be542abe10b37cd53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1835040
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

tests/git_cache_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 2015 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.

tests/git_dates_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.