Steps to Reproduce:

1. cd python-pubsub/samples/snippets
2. python3 -m venv venv && source ./venv/bin/activate
3. pip install -r requirements.txt
4. python3 -m pip install mypy
5. python3 -m mypy --install-types
6. python3 -m mypy subscriber.py
7. Observe many occurrences of error: Module "google.cloud" has no attribute "pubsub_v1" [attr-defined]