[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
4 jobs for 1.2.0 in 2 minutes and 53 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed build_3_6 #57575
docker

00:01:16

passed build_3_7 #57576
docker

00:01:16

passed build_3_8 #57577
docker

00:01:17

passed build_3_9 #57578
docker

00:01:35