Preparing v8 to use with python3 /infra

There are now less that 400 days until the end of life
of Python 2(aka _legacy_ Python) https://pythonclock.org/ .
The code compatibility check for python2 and python3
used the following tools: futurize, flake8
You can see the reports here: https://travis-ci.com/bmsdave/v8/builds

This CL was uploaded by git cl split.

Bug: v8:8594
Change-Id: Id7e2f3d5751d9f0428d28f92106748d71db0042e
Reviewed-on: https://chromium-review.googlesource.com/c/1470122Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59677}

infra/testing/PRESUBMIT.py

@@ -11,6 +11,10 @@ For simplicity, we check all pyl files on any changes in this folder.
 import ast
 import os
 
+try:
+  basestring       # Python 2
+except NameError:  # Python 3
+  basestring = str
 
 SUPPORTED_BUILDER_SPEC_KEYS = [
   'swarming_dimensions',