gsutil.vpython: Set vpython_version: 2.7

Bug: 1041020
Change-Id: I725a599dcd5dc8b2a18d49bad33b2dbdcfe7ac8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1994428
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

gsutil.vpython

@@ -11,6 +11,8 @@
 # "gsutil", where a stock system-deployed file in "/etc/" explicitly specifies
 # that the "google_compute_engine" and "boto" Python packages are available.
 
+python_version: "2.7"
+
 wheel <
   name: "infra/python/wheels/google_compute_engine-py2_py3"
   version: "version:2.6.2"