scripts: sphinx-pre-install: fix a bug when using with venv

When python3 creates a venv, it adds python into it!

This causes any upgrade recommendation to look like this:

	/devel/v4l/docs/sphinx_1.7.9/bin/python3 -m venv sphinx_2.4.4
	. sphinx_2.4.4/bin/activate
	pip install -r ./Documentation/sphinx/requirements.txt

With is wrong (and it may not work). So, when recomending
an upgrade, exclude the venv dir from the search path, and
get the system's python.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/aa622ff71bebf6960fc0262fb90e7ebc7a999a02.1587478901.git.mchehab+huawei@kernel.org


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