plugins/Sphinx: Work around Sphinx using new config format

[SmithChart]

Since Sphinx 7.3.0 they use a new toml-based config format for the theme configuration. See:
sphinx-doc/sphinx#12265

This breaks how the Flamingo Sphinx plugin handles the theme configuration.
With this change we pin Sphinx to a version <7.3.0 to work around this issue until we have a proper fix.

Also: Update changelog and bump version for a new release.

[Bastian-Krause]

Some notes from internal discussion with @SmithChart:

• Sphinx supports both ini and toml configs, but the Sphinx-internal configs flamingo parses were converted to toml (we should investigate if there's a better way than parsing these ourselves).
• We don't have test coverage for the theme config parsing, this failed for the docs jobs, though.

[Bastian-Krause]

Seems to be the easiest way to unbrick flamingo at the moment.