docs: Fix conf.py for Sphinx 2.0

docs: Fix conf.py for Sphinx 2.0

BugLink: https://bugs.launchpad.net/bugs/1838700
commit 3bc8088464712fdcb078eefb68837ccfcc413c88 upstream.

Our version check in Documentation/conf.py never envisioned a world where
Sphinx moved beyond 1.x.  Now that the unthinkable has happened, fix our
version check to handle higher version numbers correctly.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>