plugins/Sphinx: Work around Sphinx using new config format

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.

Signed-off-by: Chris Fiege <cfi@pengutronix.de>

setup.py

@@ -41,7 +41,7 @@
         'coloredlogs',
     ],
     'sphinx-themes': [
-        'sphinx>=4.5.0',
+        'sphinx>=4.5.0,<7.3.0',
         'sphinx_rtd_theme>=1.0.0',
     ],
 }