Bump opencv-python-headless from 4.2.0.34 to 4.8.1.78

Bumps [opencv-python-headless](https://github.com/opencv/opencv-python) from 4.2.0.34 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-headless
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -6,4 +6,4 @@ lxml==4.6.5
 selenium>=3.141.0
 scikit-image>=0.16.2
 deskew==0.10.2
-opencv-python-headless==4.2.0.34
+opencv-python-headless==4.8.1.78