scripts: sphinx-pre-install: Make it compatible with Python 3.6

scripts: sphinx-pre-install: Make it compatible with Python 3.6

The minimal version requirements we have is 3.9. Yet, the
script which detects it is this one. So, let's try supporting
an old version here, as we may want to suggest to upgrade
Python version to build the docs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/39d6e27a047bc3cc8208ac5e11fe6ba44faff9c4.1754992972.git.mchehab+huawei@kernel.org