fix again sphinx version, .gitlab-ci and requirements-dev

5 jobs for master in 13 minutes and 40 seconds (queued for 2 seconds)
Name Stage Failure
test-centos7 Test
python3.6 ../ graph_models ippidb --dot > source/
Running Sphinx v3.5.2

Encoding error:
'ascii' codec can't decode byte 0xc2 in position 69: ordinal not in range(128)
The full traceback has been saved in /tmp/sphinx-err-krqfni33.log, if you want to report the issue to the developers.
make: *** [html] Error 2
Cleaning up file based variables
ERROR: Job failed: exit code 1