YouTube dependency fix `youtube_dl==2020.12.2` (#6612)

Per https://github.com/ultralytics/yolov5/issues/5860#issuecomment-1035320018 by

@hdnh2006

utils/datasets.py

@@ -301,7 +301,7 @@ class LoadStreams:
             # Start thread to read frames from video stream
             st = f'{i + 1}/{n}: {s}... '
             if 'youtube.com/' in s or 'youtu.be/' in s:  # if source is YouTube video
-                check_requirements(('git+https://github.com/Cupcakus/pafy', 'youtube_dl'))
+                check_requirements(('pafy', 'youtube_dl==2020.12.2'))
                 import pafy
                 s = pafy.new(s).getbest(preftype="mp4").url  # YouTube URL
             s = eval(s) if s.isnumeric() else s  # i.e. s = '0' local webcam