check_requirements() exclude opencv-python (#3507)

Duplicate of #3495 merged to `develop`. This PR will be merged to `master`.

Fixes #3494.

hubconf.py

@@ -30,7 +30,8 @@ def _create(name, pretrained=True, channels=3, classes=80, autoshape=True, verbo
     from utils.google_utils import attempt_download
     from utils.torch_utils import select_device
 
-    check_requirements(Path(__file__).parent / 'requirements.txt', exclude=('tensorboard', 'pycocotools', 'thop'))
+    check_requirements(requirements=Path(__file__).parent / 'requirements.txt',
+                       exclude=('tensorboard', 'pycocotools', 'thop', 'opencv-python'))
     set_logging(verbose=verbose)
 
     fname = Path(name).with_suffix('.pt')  # checkpoint filename