The intersphinx_mapping in docs/conf.py is incompatible with Sphinx 8

[musicinmybrain]

Description of bug

The format of intersphinx_mapping has changed for Sphinx 8:

https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

Instead of

intersphinx_mapping = {"https://docs.python.org/3/": None}

we should write

intersphinx_mapping = {"python": ("https://docs.python.org/3/", None)}

How to reproduce

$ git checkout https://github.com/pallets-eco/flask-caching.git
$ cd flask-caching
$ python3 -m venv _e
$ . _e/bin/activate
(_e) $ pip install -e .
(_e) $ pip install -r requirements/docs.in
(_e) $ make -C docs/ html
make: Entering directory '/home/ben/src/forks/flask-caching/docs'
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v8.1.3
loading translations [en]... 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).

Expected behavior:

The documentation should build with Sphinx 8, unblocking #585.

Environment:

• Python version: Python 3.13.2
• Flask-Caching version: 494d498

Additional information:

PR to follow.