scripts: sphinx-pre-install: move package instructions to a new func

scripts: sphinx-pre-install: move package instructions to a new func

Currently, if Python < 3.7, package install will fail. That happens
with OpenSuse Leap and RHEL-based ver 8 distros.

OpenSuse allows installing Sphinx with Python 3.11, but RHEL-based
distros don't.

Prepare to recomend only venv on such cases. For now, just split
the recomendation on a new function that will check for a
paramtere to be called.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4fb2181c960e89774309a833f80209a1a3ab10d2.1754992972.git.mchehab+huawei@kernel.org