Fixes XKCD bug

comic_download/xkcd.py

@@ -51,6 +51,7 @@ class XKCDComic(Comic):
     def get_all_strips(self) -> list:
         return list(range(self.get_first_strip(), self.get_latest_strip() + 1))
 
+    @classmethod
     def __new__(cls):
         if not hasattr(cls, 'instance'):
             cls.instance = super(XKCDComic, cls).__new__(cls)