scripts/sphinx-pre-install: make activate hint smarter

It is possible that multiple Sphinx virtualenvs are installed
on a given kernel tree. Change the logic to get the latest
version of those, as this is probably what the user wants.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>