Fix signature of next()

trans/stdlib/__init__.py

-from typing import Self, TypeVar, Generic, Protocol
+from typing import Self, TypeVar, Generic, Protocol, Optional
 
 class object:
     def __eq__(self, other: Self) -> bool: ...
@@ -98,7 +98,7 @@ def iter(x: Iterable[U]) -> Iterator[U]:
 
 assert iter
 
-def next(it: Iterator[U], default: None) -> U:
+def next(it: Iterator[U], default: Optional[U] = None) -> U:
     ...
 # what happens with multiple functions