Use pyupgrade to upgrade all syntax to python 3 #15071

See comment https://github.com/googleapis/python-api-core/pull/453/files#r980307782 from googleapis/python-api-core#453, where @asottile-sentry recommended that we run pyupgrade to upgrade all syntax to python 3.

it might be a good idea to run https://github.com/asottile/pyupgrade on this codebase to remove the remaining python 2isms

We should do this on all python repositories in the googleapis org.