doc: Fix sphinx deprecation warning

Sphinx [6.2.0] and later will warn we are not using the “new” (well introduced in [1.0] in 2010) syntax:

    WARNING: The pre-Sphinx 1.0 'intersphinx_mapping' format is deprecated and will be removed in Sphinx 8. Update to the current format as described in the documentation. Hint: "intersphinx_mapping = {'<name>': ('http://docs.python.org/', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

[6.2.0]: https://www.sphinx-doc.org/en/master/changes.html#release-6-2-0-released-apr-23-2023
[1.0]: https://www.sphinx-doc.org/en/master/changes.html#release-1-0-jul-23-2010

doc/source/conf.py

@@ -258,4 +258,4 @@
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}