Build(deps): Bump opencv-python in /pymnn/test/playground

Bumps [opencv-python](https://github.com/opencv/opencv-python) from 4.1.0.25 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
...

Signed-off-by: dependabot[bot] <[email protected]>

pymnn/test/playground/requirements.txt

@@ -3,7 +3,7 @@ watchdog==0.10.6
 numpy==1.16.4
 flatbuffers==1.12
 validators==0.14.2
-opencv-python==4.1.0.25
+opencv-python==4.8.1.78
 streamlit==1.37.0
 pyrsistent==0.14.0
 boto3==1.15.17