[dart] Upgrade vpython3 and remove python 2.x remnants

Bug: b/202110580
Change-Id: I81cc02fc082091adc6eae13abe3da69926238c40
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/365000
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
12 files changed