Fix: required sphinx-bootstrap-theme version

Fix: required sphinx-bootstrap-theme version

lfdocs-conf requires sphinx-bootstrap-theme version around 0.7.1
sphinx-bootstrap-theme v0.7.1 still uses the setup command "use_2to3".
This is no more allowed with latest versions of setuptools

https://stackoverflow.com/questions/69100275/

This breaks ODL current gating.
https://jenkins.opendaylight.org/releng/job/docs-rtd-verify-phosphorus/131/
https://git.opendaylight.org/gerrit/c/docs/+/97543/1

Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>
Change-Id: I24f0b47f2898e7a9e2887e783caf36de5d0049b4