scripts/sphinx-pre-install: add '-p python3' to virtualenv

scripts/sphinx-pre-install: add '-p python3' to virtualenv

With Ubuntu 16.04 (and presumably Debian distros of the same age),
the instructions for setting up a python virtual environment should
do so with the python 3 interpreter.  On these older distros, the
default python (and virtualenv command) might be python2 based.

Some of the packages that sphinx relies on are now only available
for python3.  If you don't specify the python3 interpreter for
the virtualenv, you get errors when doing the pip installs for
various packages

Fix this by adding '-p python3' to the virtualenv recommendation
line.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Link: https://lore.kernel.org/r/1582594481-23221-1-git-send-email-tim.bird@sony.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>