Repro steps:

• make a fresh virtualenv
• run python3 -m pip install imgaug opencv-python

Want: imgaug and cv2 are both installed
Got: error during imgaug setup.py, nothing is installed.

The problem here is that setup.py is looking for cv2 to already be installed, which it may not yet be.

My use case is installing imgaug along with several other packages via pip3 install -r requirements.txt; I understand that there's an env var to skip this check, but I'd rather not start collecting env vars in every script that wants to set up an environment this way.