Mark tests as requiring setuptools in tox.ini and setup.py

test_client.py uses pkg_resources. That's part of setuptools,
not Python. Per
https://docs.python.org/3.12/whatsnew/3.12.html#removed

 , it
was previously included by default in venv environments, but no
longer is from 3.12 onwards, so we need to explicitly require
setuptools to ensure it's available.

Signed-off-by: Adam Williamson <awilliam@redhat.com>