Fix invalid Python file encoding "utf8"

Since Python 3 it must be "utf-8", which is the official name.
This is backwards compatible with Python 2.

Bug: 962277
Change-Id: I21115a322944532f1870862c8c048f8455c1109f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1608562
Auto-Submit: Raul Tambre <raul@tambre.ee>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

checkout.py

-# coding=utf8
+# coding=utf-8
 # Copyright (c) 2012 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.

patch.py

-# coding=utf8
+# coding=utf-8
 # Copyright (c) 2012 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.

subprocess2.py

-# coding=utf8
+# coding=utf-8
 # Copyright (c) 2012 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/fix_encoding_test.py

 #!/usr/bin/env python
-# coding=utf8
+# coding=utf-8
 # Copyright (c) 2011 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.