Switch coverage_report.py to use vpython.

Then provide the "six" Python package as a Python Wheel in the default
vpython environment, as the scripts in tools/code_coverage/ need it.
See https://crrev.com/893203 for reference.

Bug: pdfium:1674
Change-Id: I43597e4cf2842f2b0e6105cb56eb91597596bfdf
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/82390
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/.vpython b/.vpython
index 017ece5..d6775bd 100644
--- a/.vpython
+++ b/.vpython
@@ -36,3 +36,8 @@
   >
 >
 
+# Used by tools/code_coverage/coverage.py
+wheel: <
+  name: "infra/python/wheels/six-py2_py3"
+  version: "version:1.15.0"
+>
diff --git a/testing/tools/coverage/coverage_report.py b/testing/tools/coverage/coverage_report.py
index 71ab401..06c03e4 100755
--- a/testing/tools/coverage/coverage_report.py
+++ b/testing/tools/coverage/coverage_report.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright 2017 The PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.