update req.txt

The old 'opencv-python' package is very limiting in terms of optical flow - so I propose a package change to 'opencv-contrib-python', which has more cv2.optflow methods. 

These are needed for optical flow trickery in auto1111 and its extensions, and it cannot be installed by an extension as only a single package of opencv needs to be installed for optical flow to work properly. Change of the main one is Inevitable.

requirements.txt

@@ -9,7 +9,7 @@ gradio==3.15.0
 invisible-watermark
 numpy
 omegaconf
-opencv-python
+opencv-contrib-python
 requests
 piexif
 Pillow