The infrastructure runs everything already in Python3, so this is
mostly a clean-up.

For MB, a python2 holdover was removed and new lint errors were
fixed.

The renames were automated with:
git grep -e "/usr/bin/python$" |
  cut -d':' -f1 |
  xargs
  sed -i 's/#!\/usr\/bin\/python$/#!\/usr\/bin\/python3/1'

and
git grep -e "/usr/bin/env python$" |
  cut -d':' -f1 |
  xargs
  sed -i 's/#!\/usr\/bin\/env python$/#!\/usr\/bin\/env python3/1'

Bug: v8:13148
Change-Id: If4f3c7635e72fa134798d55314ac1aa92ddd01bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3811499Reviewed-by: Liviu Rau <liviurau@google.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82231}