Documentation: update packages?

[sk1p]

It looks like our documentation dependencies in combination currently force installation of a quite old version of sphinx (5.x, current version is 7.x) , and also our theme used (sphinx-rtd-theme==0.5.1, current version 1.x).

(see logs for a recent docs build here, for example: https://github.com/LiberTEM/LiberTEM/actions/runs/6233324633/job/16918297835)

For now this is a combination that seems to work, but we need to keep an eye on this, and maybe at some point find out what is causing the issue, and get things up to date again.

A similar issue can be seen in LiberTEM-live, where the outdated sphinx-rtd-theme causes the build to fail if we don't additionally constrain the sphinx version to <7... LiberTEM/LiberTEM-live#131 - this should not be necessary, as the latest version of the theme does support the current sphinx version.

[sk1p]

sphinx is still constrained to <7.3, which is already better than the old constraint. I'll check in #1638 if we can remove that constraint, too.

[sk1p]

Still fails with current sphinx, I'll revert:

/home/runner/work/LiberTEM/LiberTEM/docs/source/app/processing.rst:22: WARNING: domain for external cross-reference not found: 'ptychography40'
WARNING: cannot cache unpickable configuration value: 'nbsphinx_custom_formats' (because it contains a function, class, or module object)