Bump opencv-python from 4.3.0.38 to 4.4.0.40 in /cvat/requirements (#2021)

Bumps [opencv-python](https://github.com/skvark/opencv-python) from 4.3.0.38 to 4.4.0.40.
- [Release notes](https://github.com/skvark/opencv-python/releases)
- [Commits](https://github.com/skvark/opencv-python/commits)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

cvat/requirements/base.txt

@@ -40,7 +40,7 @@ matplotlib==3.0.3
 scikit-image==0.15.0
 tensorflow==2.2.0
 keras==2.4.2
-opencv-python==4.3.0.38
+opencv-python==4.4.0.40
 h5py==2.10.0
 imgaug==0.4.0
 django-cors-headers==3.4.0