Drop Sphinx source_suffix config

The deprecated list of strings appears to have been removed in
Sphinx=8. The default `source_suffix = {'.rst': 'restructuredtext'}` is
appropriate for this project.

docs/conf.py

@@ -87,9 +87,6 @@
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
 
-# The suffix of source filenames.
-source_suffix = '.rst'
-
 # The master toctree document.
 master_doc = 'index'