scripts: sphinx-pre-install: add support for OpenMandriva

It seems that Mageia and OpenMandriva will reunite on a single
distribution. In any case, both came from Mandriva. So, it is
close enough to use the same logic.

So, add support for it.

Tested with OpenMandriva 4.1 and with Mageia 7.1.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/692809729c6818a0b0f75513da15970c53d5565c.1586883286.git.mchehab+huawei@kernel.org


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