Fix `www.youtube.com` hostname (#7242)

* Fix `www.youtube.com` hostname

* Update datasets.py

utils/datasets.py

@@ -316,7 +316,7 @@ class LoadStreams:
         for i, s in enumerate(sources):  # index, source
             # Start thread to read frames from video stream
             st = f'{i + 1}/{n}: {s}... '
-            if urlparse(s).hostname in ('youtube.com', 'youtu.be'):  # if source is YouTube video
+            if urlparse(s).hostname in ('www.youtube.com', 'youtube.com', 'youtu.be'):  # if source is YouTube video
                 check_requirements(('pafy', 'youtube_dl==2020.12.2'))
                 import pafy
                 s = pafy.new(s).getbest(preftype="mp4").url  # YouTube URL