py3: add instruction on how to enable py3 for led build
py3: add instruction on how to enable py3 for led build

R=iannucci, yuanjunh

Change-Id: I0562ea6208a8997d201dac0c024e7b57d4e9fb50
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3135095
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py