This file is used by a few dependent projects. Highlighting py3
compatiblity eases the migration process for those projects.

This CL additionally adds executation permissions to the file and
includes the script in the docs.

Bug: 1298903
Change-Id: I5838fedcef181ee5e3eb84a1fa16b649250460ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3479854
Auto-Submit: Alexander Schulze <alexschulze@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>