An increasing number of Windows developers are "accidentally" ending up
with Python 3 installed on their machine through the Windows Store. If
this version is first in the path then gclient may fail in non-obvious
ways. This adds a python version check to give a clear warning message.

R=vapier@chromium.org

Change-Id: I7a292fba2ab7d059a6149644d0b1fd92d2371dce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1825945
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>