scripts: sphinx-pre-install: place a warning for Sphinx >= 3.0

scripts: sphinx-pre-install: place a warning for Sphinx >= 3.0

Newer versions of Sphinx have a known bug:
	https://github.com/sphinx-doc/sphinx/pull/8313

Those currently produces 11 false-positive warnings On Sphinx
version 3.1+.

While Sphinx fix is not applied, let's warn adventurers that would
be using Sphinx installed via distro packages.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/085e0ada65c11da9303d07e70c510dc45f21315b.1656756450.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>