[reddit] set default 0.6s delay between requests (#4292)

to limit API requests to 100 per minute
https://www.reddit.com/r/redditdev/comments/14nbw6g/

gallery_dl/extractor/reddit.py

@@ -20,6 +20,7 @@ class RedditExtractor(Extractor):
     filename_fmt = "{id}{num:? //>02} {title[:220]}.{extension}"
     archive_fmt = "{filename}"
     cookiedomain = ".reddit.com"
+    request_interval = 0.6
 
     def items(self):
         self.api = RedditAPI(self)