add opencv-python to requirements.txt (#2061)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Updated project install-time dependencies to include OpenCV
(opencv-python), ensuring the runtime now bundles OpenCV’s Python
bindings.
* Dependency-only change; no public interfaces or exported entities were
modified.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: guozijn <guozijn@gmail.com>

Author: guozijn

detection/requirements.txt

@@ -2,3 +2,4 @@ monai-weekly[nibabel]
 itk>=5.2; python_version < "3.10"
 tensorboard
 torchvision
+opencv-python