[recipes] Python3 changes.

This:
  * Updates out of date depot_tools and recipe_engine dependencies
  * Updates out of date recipes.py
  * Sets `py3_only` flag in recipes.cfg

Bug: 1298328
No-Try: true
Change-Id: Ib5eb22cf834cf542c5007e162c9efe0f40e33807
Reviewed-on: https://chromium-review.googlesource.com/c/codecs/infra/+/4351389
Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
21 files changed