Strip surrounding whitespace in filenames

Such spaces are annoying enough to deal with, and break external downloaders trying to be smart (such as aria2c).
Fixes #20312.

youtube_dl/utils.py

@@ -509,7 +509,7 @@ def sanitize_filename(s, restricted=False, is_id=False):
     if not is_id:
         while '__' in result:
             result = result.replace('__', '_')
-        result = result.strip('_')
+        result = result.strip(' _')
         # Common case of "Foreign band name - English song title"
         if restricted and result.startswith('-_'):
             result = result[2:]