replace old %-formatted and .format(…) strings with f-strings (#7671)

mostly using flynt
https://github.com/ikamensh/flynt

gallery_dl/extractor/yiffverse.py

@@ -46,8 +46,8 @@ class YiffverseExtractor(BooruExtractor):
 
         post_id = post["id"]
         root = self.root_cdn if files[fmt][0] else self.root
-        post["file_url"] = url = "{}/posts/{}/{}/{}.{}".format(
-            root, post_id // 1000, post_id, post_id, extension)
+        post["file_url"] = url = \
+            f"{root}/posts/{post_id // 1000}/{post_id}/{post_id}.{extension}"
         post["format_id"] = fmt
         post["format"] = extension.partition(".")[0]
 
@@ -73,11 +73,11 @@ class YiffverseExtractor(BooruExtractor):
             post["tags_" + types[type]] = values
 
     def _fetch_post(self, post_id):
-        url = "{}/api/v2/post/{}".format(self.root, post_id)
+        url = f"{self.root}/api/v2/post/{post_id}"
         return self.request(url).json()
 
     def _pagination(self, endpoint, params=None):
-        url = "{}/api{}".format(self.root, endpoint)
+        url = f"{self.root}/api{endpoint}"
 
         if params is None:
             params = {}