docs: Fix deprecated `intersphinx_mapping` format

[Mr0grog]

While working on #771, I noticed that Sphinx is logging a warning on builds because the format being used for intersphinx_mapping is deprecated and will be removed in the next major release of Sphinx:

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>': ('https://docs.python.org/', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

This updates the format for intersphinx_mapping to use the newer, actively maintained format. (I’m not this matters too much, though, since there are no type annotations and none of the docstrings list types for intersphinx to make links out of. 🤷)

[Mr0grog]

Now that someone’s allowed tests to run on this PR, there seem to be a lot of failures. FWIW, this works well for me locally and I don’t think the changes here would have caused any test failures. Not sure if there’s anything I should be doing, or if this is just a CI configuration issue for PRs from forks, or something else.