client: add embedded vpython3 spec in swarming.py
client: add embedded vpython3 spec in swarming.py

This is to re-land
https://crrev.com/c/3853021

Bug: 1327534
Change-Id: Iac35dd4b48d5413454ec36c3540c22c89d6f3ee8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3855656
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py