This CL adds support for running PRESUBMIT.py under either Python2
or Python3 as specified in each PRESUBMIT.py file.

To run the checks under Python3, the PRESUBMIT.py file must contain
a line exactly matching "^USE_PYTHON3 = True$". If the file
does not contain this string, the checks will run under Python2.

Different PRESUBMIT.py files in a single CL may thus contain
a mix of 2- and 3-compatible checks, but each individual file will
only be run in one or the other (it doesn't likely make sense to run
them in both by default).

Bug: 1157663
Change-Id: Ic74977941a6519388089328b6e1dfba2e885924b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2832654
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>