Currently, post upload presubmit hooks are exlusively executed with py2,
regardless of USE_PYTHON3 magic variable. This change adds py3 support
in the same fasion as regular presubmit hooks.

R=aravindvasudev@google.com

Fixed: 1297712
Change-Id: Ib464f8563e4135a63fc48692d27c8692fe1f630b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3469285Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>