fix: Pin Sphinx<9.0.0 for sphinx-tabs compatibility

fix: Pin Sphinx<9.0.0 for sphinx-tabs compatibility

Sphinx 9.0.0 introduced breaking changes that cause sphinx-tabs
to fail with KeyError: 'backrefs' in tabs.py line 69.

This affects projects like bgpcep where tox docs builds fail:
https://jenkins.opendaylight.org/releng/job/bgpcep-tox-verify-master/1719/console

Root cause: sphinx-tabs has not been updated for Sphinx 9.x yet.
See: https://github.com/executablebooks/sphinx-tabs/issues/206

Pin Sphinx to <9.0.0 until sphinx-tabs releases a compatible version.

Issue-ID: IT-29177
Change-Id: Ic69a55380709003c0a9911c9bf29f374a1d2f0a9
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>