PRESUBMIT: drop python2 usage

We haven't used Python 2 in years.

BUG=None
TEST=CQ passes

Change-Id: I08924e67386a7a499c55a2ea90ebd6b2d7ac2caa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/flashmap/+/4240702
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>