scripts: sphinx-pre-install: some adjustments related to venv

scripts: sphinx-pre-install: some adjustments related to venv

While nothing was really needed for virtualenv to work on most
distros, we had an issue with OpenMandriva.

While checking for it, it was noticed that there was no check if
python-virtualenv was installed.

This didn't solve the issues we faced there: at least with
the half-broken OpenMandriva Lx 4.0 docker container we used,
ensurepip was not available anywhere, causing venv to fail.

Add a distro-specific note about that.

Note: at least at the time we did our tests, OpenMandriva Lx 4.0
docker was shipped with wrong dnf repositories. Also, there
was no repos available for it anymore. So, we had to do some
hacks to upgrade to 4.3 before being able to run any tests.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/e3a0e5eccd50eb506846e3e8487a2d9124ef83e2.1754992972.git.mchehab+huawei@kernel.org