[nova] python3.9 support

Hi, I run unit and functional tests (where applicable) for nova, python-novaclient, os-vif, placement, osc-placement, os-traits, os-resource-classes with python 3.9 and everything is green. I've proposed some tox.ini targets where it was needed [1][2][3] Cheers, gibi [1]https://review.opendev.org/#/c/760884/ [2]https://review.opendev.org/#/c/760890/ [3]https://review.opendev.org/#/c/760912/