Sphinx warning: deprecated syntax for intersphinx_mapping

[lmoureaux]

Describe the bug

sphinx 7.2.6 warns:

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

To Reproduce
Steps to reproduce the behavior:

1. Build on Arch
2. Look out for Sphinx messages

Expected behavior
We don't use deprecated syntax.

Platform and version (please complete the following information):

• OS: Arch Linux
• Freeciv21 version: master, stable likely affected too