sphinx: conf.py: properly handle Sphinx 4.0

One of the checks for Sphinx 3+ is broken, causing some
C warnings to return back with Sphinx 4.0.x.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d5abc30056dafeec0778a46263a45401bdc7f11e.1603791716.git.mchehab+huawei@kernel.org


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