docs: requirements.txt: recommend Sphinx 1.7.9

As discussed at the linux-doc ML, while we'll still support
version 1.3, it is time to recommend a more modern version.

So, let's switch the minimal requirements to Sphinx 1.7.9,
as it has the "-jauto" flag, with makes a lot faster when
building documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>