Tests/CI broken on Python 3.12

[mportesdev]

Python 3.12 does not include setuptools automatically in a virtual environment. As a result, running tox run -e py312 fails (with a weird message stestr: 'run' is not a stestr command.)

After installing setuptools manually, running tox run -e py312 errors with ModuleNotFoundError: No module named 'imp'. The imp module is removed in Python 3.12, however it is required by the future package, which in turn is required by stestr.

[mportesdev]

The latter seems to be already addressed here mtreinish/stestr@112598d (but not included in a published release yet).