Support `sphinx` 8

[jtpio]

The latest Sphinx 8 seems to be causing issues for the docs build, as seen in #123.

https://readthedocs.org/projects/voici/builds/25309076/

Running Sphinx v8.0.2
loading translations [en]... done
making output directory... done
Converting `source_suffix = '.md'` to `source_suffix = {'.md': 'restructuredtext'}`.
ERROR: Invalid value `None` in intersphinx_mapping['https://docs.python.org/']. Expected a two-element tuple or list.

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/voici/conda/123/lib/python3.12/site-packages/sphinx/cmd/build.py", line 332, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/voici/conda/123/lib/python3.12/site-packages/sphinx/application.py", line 277, in __init__
    self.events.emit('config-inited', self.config)
  File "/home/docs/checkouts/readthedocs.org/user_builds/voici/conda/123/lib/python3.12/site-packages/sphinx/events.py", line 100, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/voici/conda/123/lib/python3.12/site-packages/sphinx/ext/intersphinx/_load.py", line 127, in validate_intersphinx_mapping
    raise ConfigError(msg)
sphinx.errors.ConfigError: Invalid `intersphinx_mapping` configuration (1 error).

Configuration error:
Invalid `intersphinx_mapping` configuration (1 error).

#123 pinned to sphinx<8 for now, but we should look into supporting the latest version.