RHEL 7.x and clone distros are shipped with Sphinx 1.1.x,
with is incompatible with Kernel ReST markups.

So, on those systems, the only alternative is to install
it via a Python virtual environment.

While seeking for "pip" on CentOS 7.3, I noticed that it
is not really needed, as python-virtualenv has its version
packaged there already. So, remove this from the list of
requirements for all distributions.

With regards to PDF, we need at least texlive-tabulary
extension, but that is not shipped there (at least on
CentOS). So, disable PDF packages as a whole.

Please notice, however, that texlive + amsmath is needed for
ReST to properly handle ReST ".. math::" tags. Yet, Sphinx
fall back to display the LaTeX math expressions as-is, if
such extension is not available.

So, let's just disable all texlive packages as a whole.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>