[mod] Move sphinx from 3.8 to 3.9 image

Since DuniterPy switch the main version to 3.9
Fix Sphinx version to 3.x, since v4.x breaks the doc
1 job for 1_drop_3.5_pre-commit in 1 minute and 51 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed test_build_image #57574
docker

00:01:51