Add stub for string.casefold

Exists in 3.3+

stdlib/3/builtins.pyi

@@ -233,6 +233,7 @@ class str(Sequence[str]):
     @overload
     def __init__(self, o: bytes, encoding: str = ..., errors: str = 'strict') -> None: ...
     def capitalize(self) -> str: ...
+    def casefold(self) -> str: ...
     def center(self, width: int, fillchar: str = ' ') -> str: ...
     def count(self, x: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def encode(self, encoding: str = 'utf-8', errors: str = 'strict') -> bytes: ...