Switch jobs to python3
Switch jobs to python3

- ironic-base is python3 by default, we don't need `USE_PYTHON3`

Depends-On: https://review.opendev.org/#/c/687521/
Change-Id: I75ee04332e11737c3fa4fb429a9a306a6b05b011

openstack/python-ironicclient,openstack/python-ironicclient