Dependencies: update requirement sphinx~=4.0

[sphuber]

Fixes #5373

The builds on ReadTheDocs started failing. This was due to release v2.1
of markupsafe that removed the module markup_safe. The version of
jinja==2.11.3 that was getting installed still imported this. This
version is no longer supported and so they won't be fixing this
incompatibility but instead one should use jinja>3.0. This is already
a requirement for aiida-core, but the version of sphinx~=3.2.1 that
we were installing had an upper limit on jinja to not install v3
causing the version of jinja to be downgraded. The solution is to update
the version of sphinx which no longer has this limitation of jinja.

Finally, we need to require sphinx~=4.1, because v4.0.x puts an
upper limit markupsafe<2.0 which is incompatible with jinja~3.0 but
this upper limit was removed later on.

[sphuber]

Dependency hell 😢 think this solves the dependency requirement mess, but in upgrading sphinx, there are now 800 freaking warnings in the docs for some reason. WIll have to look into those later

[chrisjsewell]

but in upgrading sphinx, there are now 800 freaking warnings in the docs for some reason.

You don't think I'd tried this before, and aborted for the same reason 😅

[sphuber]

With sphinx~-4.0 we are getting the following class of new warnings that fail the build because we use the nitpick option:

1. WARNING: more than one target found for cross-reference: most if not all of these are simply our own aliases in module init files such as aiida.manage.Config, aiida.manage.configuration.config.Config, aiida.manage.configuration.Config. For some reason Sphinx v4 no longer understands these aliases. Seems to be an unsolved issue. See [1][2]
2. WARNING: py:class reference target not found These include external as well as internal references.
3. WARNING: duplicate object description of aiida.tools.data.orbital.orbital.Orbital, other instance in reference/apidoc/aiida.tools.data, use :noindex: for one of them quick scan seems to suggest that it basically is just the reference/apidoc folder being the problem, which is the autogenerated API docs through sphinx-apidoc.

[sphuber]

Closing this for now