Pin Sphinx version to fix failing builds

[MarekSzczypinski]

Description
New version of Sphinx and docutils make the builds fail.
Sphinx>=9.0.0 is requiring the definition of default_role in config.py and the docutils>=22.0.0 generate different than expected man pages from existing *.rst files causing the tests of documentation to fail.

Problem happens for Python >= 3.11 so updating the man pages would in turn break the builds/tests for Python 3.9 and 3.10 that are currently passing.

My proposition is to pin Sphinx package version to one that works with all labgrid-supported Python versions. Doing that also (indirectly) pins version of docutils package to one that doesn't alter generation of man pages.

The solution is temporary until we're able to drop support for python 3.9 and 3.10.

Checklist

• Documentation for the feature
• Tests for the feature

• The arguments and description in doc/configuration.rst have been updated

• Add a section on how to use the feature to doc/usage.rst

• Add a section on how to use the feature to doc/development.rst

• PR has been tested

• Man pages have been regenerated

Fixes #1801