Environment details

• OS: OSX, Linux
• Python version: 3.7
• pip version:
• google-auth version: 1.24.0

Steps to reproduce

1. Setup a clean virtualenv
2. pip install google-auth==1.24.0 aiohttp==3.5.4 (version I happen to be using)
3. Run "from google.auth.transport import _aiohttp_requests" in a python3 session

You should get a ModuleNotFoundError for "urllib3"