Newer builds of vpython contain vpython3 as well.

Bug: 898348
Change-Id: I63b197690e2ae47bd4c6cd50894c4769e1a18334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1727581Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: smut <smut@google.com>