Bump opencv-python in /official/projects/unified_detector

Bumps [opencv-python](https://github.com/opencv/opencv-python) from 4.2.0.32 to 4.8.1.78.
- [Release notes](https://github.com/opencv/opencv-python/releases)
- [Commits](https://github.com/opencv/opencv-python/commits)

---
updated-dependencies:
- dependency-name: opencv-python
  dependency-type: direct:production
...

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