Solve dependency conflict between Flake8 and Sphinx

[Cito]

Currently we cannot upgrade to Sphinx v5, because Sphinx and Flask are installed together in the dev environment, and the current versions Sphinx 5 and Flask 4 cannot coexist (see for instance here).

The best solution will be to use dependency groups to separate at least the "docs" environment from the "dev" environment. This is something that is supported and recommended in the next version 1.2 of Poetry.

I think we should do that when Poetry 1.2 has been released and Sphinx 5 is more widely used (to be sure that the themes and "read the docs" support it).

[Cito]

This may be solved with Flask 5, but it would still be a good idea. Will tackle this in the next minor version where we require Python 3.7 because Poetry 1.2 needs that.

[Cito]

Unfortunately, Flask 5 and Sphinx 5 still have conflicting dependencies.

And unfortunetely, it turend after switching to dependency groups in the development branch for v3.3, this did not solve the problem. Poetry still tries to conciliate all dependency groups, even those that are not installed, and there is no fix for this problem planned in Poetry - see this issue in Poetry.

So I'm looking for a different solution to solve this in the 3.3 branch. Essentially, we require these dependency groups to be installed in different virtual envs, but I'm not sure how I can express this in the pyproject.toml. Maybe these dependencies should simply not go there, but in the tox or nox configuration.

[Cito]

For now, I have just relaxed the dependencies in pyproject.toml (allowing Sphinx 4.3), and require Sphinx 5.1 only in tox.ini.

I think this is good enough, and that the dependency conflict will eventually disappear anyway, so closing this now.