khal fails to build with sphinx 8.x in Fedora 42 (currently Rawhide).

The issue detected:
```
ERROR: Invalid value `None` in intersphinx_mapping['http://docs.python.org/']. Expected a two-element tuple or list.

Configuration error:
Invalid `intersphinx_mapping` configuration (1 error).
make: *** [Makefile:131: man] Error 2
```

The intersphinx_mapping error can be easily fixed by adjusting conf.py.
You can take an inspiration from this PR: https://github.com/jupyterlab/jupyterlab/pull/14423

Please work with upstream on this issue, I plan to ship sphinx 8 in two weeks.


For the logs from testing build attempts, see:
https://copr.fedorainfracloud.org/coprs/thrnciar/python-sphinx/package/khal/

You can test you package in mock running:

$ mock -r fedora-rawhide-x86_64
--addrepo=https://download.copr.fedorainfracloud.org/results/thrnciar/python-sphinx/fedora-rawhide-x86_64/ --no-clean your.src.rpm
$ mock -r fedora-rawhide-x86_64
--addrepo=https://download.copr.fedorainfracloud.org/results/thrnciar/python-sphinx/fedora-rawhide-x86_64/ shell

We'd like to include Sphinx 8 in Fedora 42.
Let us know here if you have any questions. Thank you!