The python module is deprecated in favor of using the step module with
an explicit python binary. Switching the code to use the step module and
remove DEPS on recipe_engine/python removes warnings when running recipe
tests in the repo.

Change-Id: If3c9d5d6aff5aab69898b535399accbef92f5c0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3880624
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>