hterm: vtscope: port to Python 3
hterm: vtscope: port to Python 3

Only lightly tested.  We need to get off Python 2 at any rate,
and our local pylint script no longer finds issues with it.

Change-Id: Ifea5332f24081303ca007a6eeb5798caade34265
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1808318
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm