Setting PYTHON_VERSION_COMPATIBILITY = PY2+3 makes "recipe.py test" run
tests in both py2 and py3.

Bug: 1227140
Change-Id: I56a47f0491cbade172e1dc1239cbf6c2a0faa64f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3116850
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>