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

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

gallery_dl/extractor/bbc.py

@@ -43,7 +43,7 @@ class BbcGalleryExtractor(GalleryExtractor):
     def images(self, page):
         width = self.config("width")
         width = width - width % 16 if width else 1920
-        dimensions = "/{}xn/".format(width)
+        dimensions = f"/{width}xn/"
 
         results = []
         for img in text.extract_iter(page, 'class="gallery__thumbnail', ">"):
@@ -64,7 +64,7 @@ class BbcGalleryExtractor(GalleryExtractor):
         front, _, back = src.partition("/320x180_b/")
         for width in (1920, 1600, 1280, 976):
             if width < max_width:
-                yield "{}/{}xn/{}".format(front, width, back)
+                yield f"{front}/{width}xn/{back}"
 
 
 class BbcProgrammeExtractor(Extractor):