fix: Port to Sphinx 8.0 (#509)

The old `intersphinx_mapping` format has been removed; it must now map
identifiers to (target, inventory) tuples.

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: cjwatson

docs/conf.py

@@ -179,4 +179,4 @@
 # -- Options for intersphinx extension ---------------------------------------
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {"https://docs.python.org/3/": None}
+intersphinx_mapping = {"python": ("https://docs.python.org/3/", None)}