This change also makes CheckVPythonSpec() look for .vpython3 files.

Bug: 1336295
Change-Id: I4bbec48debe42748811a8cfcef6da9602017d4bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3891974Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Fabrice de Gans <fdegans@chromium.org>