goma_link_tests: Require Python 3

tools/clang/scripts/goma_link_tests.py was originally written to work
with both Python 2 and Python 3. I intend to add some unit tests that
will use unittest.mock, which is not part of Python < 3.3. Instead of
complicated workarounds, let's just a modern Python version for the
tests.

Bug: 877722
Change-Id: I475091b005e1f2e4b804e1c884d255ff60669847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140598Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757507}

tools/clang/scripts/goma_link_tests.py

-#! /usr/bin/env python
+#! /usr/bin/env python3
 # Copyright (c) 2020 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.
@@ -17,11 +17,6 @@
 #
 # The report will be available as htmlcov/index.html
 
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 import goma_ld
 import goma_link