Python3 migration: add .vpython3
Python3 migration: add .vpython3

This is copied from chromium/src, given we've been using this version
for our migration without even realizing it

Bug: chromium:1198408
Change-Id: I92e6398a0ca38b20e0fcf473f397290278154d24
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/2912086
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Commit-Queue: Ryan Heise <50ee0a56d43e9e15503d071e9ceb1a6059353025@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult