fix(google-auth): Drop support for Python 3.8 and 3.9#16946

parthea merged 8 commits intomaingoogleapis/google-cloud-python:mainfrom

feat/drop-python-3.7-3.9-google-authgoogleapis/google-cloud-python:feat/drop-python-3.7-3.9-google-authCopy head branch name to clipboard