remove @staticmethod decorators

There might have been a time when calling a static method was faster
than a regular method, but that is no longer the case. According to
micro-benchmarks, it is 70% slower in CPython 3.13 and it also makes
executing the code of a class definition slower.

gallery_dl/formatter.py

@@ -463,8 +463,7 @@ class Literal():
     # __getattr__, __getattribute__, and __class_getitem__
     # are all slower than regular __getitem__
 
-    @staticmethod
-    def __getitem__(key):
+    def __getitem__(self, key):
         return key