This modernizes python code without breaking Py2 compat.

Ran with command:

futurize --stage1 -w tools/testrunner/testpro

Manual fixup to util_unittest to modify import paths and change to
absolute imports.

Bug: v8:9871
Change-Id: I2ac29622aff5daebc9dc42145c1d96dff8258546
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2252549Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68492}