Similarly to how python3 is available at python-bin/, this will allow
developers to opt-in to a depot_tools provided python2.

Bug: 1109180
Change-Id: I872eaa1289bb477da73373e99f5e6188a87d39dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2512906Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>