doc: update sphinx config to work with newer sphinx

[keszybz]

The config didn't work with Sphinx 8.x:

$ make html
Running Sphinx v8.1.3
WARNING: Calling get_html_theme_path is deprecated. If you are calling it to define html_theme_path, you are safe to remove that code. WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English). loading translations [en]... done
making output directory... done
Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`. ERROR: Invalid value `None` in intersphinx_mapping['https://docs.python.org/3/']. Expected a two-element tuple or list.

Configuration error:
Invalid `intersphinx_mapping` configuration (1 error). make: *** [Makefile:20: html] Error 2