scripts/sphinx-pre-install: add minimum support for RHEL

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>

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