A bug in the logic enabling tests to be run on python3 resulted in
tests never being run in python3 unless they also have a shebang line
pointing to python3. Since this bug was introduced, effectively, most
tests that are supposed to be run on python3 do not run at all. Since
then a lot of python3 tests have regressed in Chromium.

In order to fix the issue moving forward, this introduces a new
argument that skips the shebang check to run a test in python3. Tests
will be re-enabled one by one until every single instance in Chromium
has been updated.

Bug: 1223478
Change-Id: I91a0688c6f4d9b4fbf18e3d446366cded8c7f2f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2986400Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Fabrice de Gans <fdegans@chromium.org>