ci: update to Python 3 (#93)
ci: update to Python 3 (#93)

Use Python 3 in the builds as Python 2.7 reached EOL. We only need
Python because we reformat the CMake files using cmake-format.
googleapis/google-cloud-cpp-pubsub,googleapis/google-cloud-cpp-pubsub,googleapis/google-cloud-cpp-pubsub,googleapis/google-cloud-cpp-pubsub