Split `test` dependencies into `dev` and `test` dependencies.

[picnixz]

When (and if) PEP 735 is accepted, we can probably simplify the deduplication of packages. It would at least be cleaner that way.

This does not resolve #12339 because we still need to change the pyproject and tox.ini of sphinxcontrib-* packages so that they use sphinx[dev] and not just sphinx.