updated opencv dependencies (#659)

contrib/crowd_counting/requirements.txt

@@ -2,7 +2,7 @@ h5py==2.8.0
 flask==1.0.2
 scikit-image
 bokeh
-opencv-python==3.4.2.17
+opencv-python>=4.2.0.32
 pillow
 Cython
 contextlib2

contrib/crowd_counting/setup.py

@@ -8,6 +8,6 @@ setup(
     author_email="lxzhang@gmail.com",
     packages=find_packages(),
     install_requires=[
-        "h5py==2.8.0", "flask==1.0.2", "scikit-image", "bokeh", "opencv-python==3.4.2.17", "pillow", "Cython", "contextlib2"
+        "h5py==2.8.0", "flask==1.0.2", "scikit-image", "bokeh", "opencv-python>=4.2.0.32", "pillow", "Cython", "contextlib2"
     ]
 )