We moved chromite to require python 3.6 all the time, so update
the wrappers to match.

Bug: 997354
Test: `cros_sdk` still works
Change-Id: I78f5dc101f98a074fdc92a1f270395e2d04a2ea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2324825Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>