Fix bug with Imgur gifs being shortened too much

The rstrip function was used wrongly, it doesn't remove a substring but
rather removes any of the characters provided, so here it removed any I,
G, V, or F that finished the six character ID for Imgur, resulting in a
404 error for the resources in question.

bdfr/site_downloaders/imgur.py

@@ -39,7 +39,7 @@ class Imgur(BaseDownloader):
     def _get_data(link: str) -> dict:
         if re.match(r'.*\.gifv$', link):
             link = link.replace('i.imgur', 'imgur')
-            link = link.rstrip('.gifv')
+            link = re.sub('\\.gifv$', '', link)
 
         res = Imgur.retrieve_url(link, cookies={'over18': '1', 'postpagebeta': '0'})