scripts: sphinx-pre-install: fix the need of virtenv packages

The pip packages are only needed when the distro-provided
Sphinx version is not good enough.

Don't recommend installing it if not needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/04ce53b77b37f1e495c3abc39c2d3dc407895dc0.1621949137.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>