I'm now able to build the Python documentation with Sphinx 3.2.1 without modifying the Doc/Makefile, so using -W option (treat warnings as errors): there are no more Sphinx 3 warnings.

When Sphinx 3 will be more widely available (ex: in Linux distributions), we will be able to consider removing c_allow_pre_v3=True and c_warn_on_allowed_pre_v3=False in doc/conf.py. For that, we should update the documentation to use the Sphinx 3 syntax, see PR 19397 written by Jakob Lykke Andersen.

For now, I prefer to keep Sphinx 2 support and so keep Sphinx 2 syntax in the C domain, since it allows supporting Sphinx 2 and Sphinx 3.