sphinx-pre-install: use a requirements file

Instead of using 3 commands to install a virtualenv, use
a single one, reading the requirements from this file:

	Documentation/sphinx/requirements.txt

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
2 files changed