Sync sphinx dependency with global requirements. It caps python 2 since sphinx 2.0 no longer supports Python 2.7.
This issue was fixed in the openstack/watcher-tempest-plugin 1.2.0 release.
The recent review which introduced PDF building seems to be have fixed the requirements:
https://review.opendev.org/#/c/668823/
See https://review.opendev.org/#/c/668823/ (there are no releases which contain the fix at the moment though)
The fix has been part of tempest since 22.0.0 release.
Bug watches keep track of this bug in other bug trackers.
This issue was fixed in the openstack/ watcher- tempest- plugin 1.2.0 release.