scripts: sphinx-pre-install: only ask to activate valid venvs

If a venv doesn't contain Sphinx, or has an older Sphinx
version, ignore it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d11a00d88514e8a0357e1b0a05ebd518952a1d39.1587478901.git.mchehab+huawei@kernel.org


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