[all] DevStack jobs broken due to setuptools not available for Python 2

Hi all, I noticed DevStack jobs fail all over the place [1] due to: UnsupportedPythonVersion: Package 'setuptools' requires a different Python: 2.7.17 not in '>=3.5' Bug reported in [2]. Notice USE_PYTHON3=True does not help as stack.sh is hardcoded to versionless Python. [1] https://zuul.opendev.org/t/openstack/builds?result=RETRY_LIMIT&result=FAILURE [2] https://bugs.launchpad.net/devstack/+bug/1859350 -yoctozepto