python-sphinxtesters fails to build with Sphinx 7.x in Fedora 39 (currently Rawhide).

The issue detected:
```
+ /usr/bin/python3 setup.py build_sphinx -b html

usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
or: setup.py --help [cmd1 cmd2 ...]
or: setup.py --help-commands
or: setup.py cmd --help

error: invalid command 'build_sphinx'
```

Sphinx 7 has removed the setuptools integration (see: https://github.com/sphinx-doc/sphinx/pull/11363).
`setup.py` has been long deprecated and projects are encouraged to migrate away from using it.


For the logs from testing build attempts, see:
https://copr.fedorainfracloud.org/coprs/ksurma/doc-stack/package/python-sphinxtesters/

You can test you package in mock running:

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

We'd like to include Sphinx 7 in Fedora 39.
Let us know here if you have any questions. Thank you!