Bump opencv-python from 4.0.0.21 to 4.8.1.78

Bumps [opencv-python](https://github.com/opencv/opencv-python) from 4.0.0.21 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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Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
-opencv_python==4.0.0.21
+opencv_python==4.8.1.78
 numpy==1.15.2
 pandas==0.23.4
 torch==0.4.1