grouping digits with underscores

More readable. Since Python 3.6: https://docs.python.org/3/library/functions.html#int

Co-authored-by: pukkandan <pukkandan.ytdlp@gmail.com>

yt_dlp/utils/_utils.py

@@ -1382,7 +1382,7 @@ def datetime_round(dt_, precision='day'):
     if precision == 'microsecond':
         return dt_
 
-    time_scale = 1000000
+    time_scale = 1_000_000
     unit_seconds = {
         'day': 86400,
         'hour': 3600,