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

Per ultralytics#5860 (comment) by @hdnh2006

utils/datasets.py

@@ -301,7 +301,7 @@ def __init__(self, sources='streams.txt', img_size=640, stride=32, auto=True):
             # 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