[PY-647][external]Fixed cv2 import statement

[JBWilkie]

Problem

When pulling video files as frames, we use cv2 to extract & write frames. A small bug prevents darwin-py from accessing the necessary imwrite cv2 function to do this, resulting in ImportError

Solution

Fixed the import statement & function reference

Changelog

Fixed issue with pulling videos as frames

[linear]

PY-647 ImportError when pulling videos as frames