Make BaseException.__new__ cls positional-only (#16066)

Author: carljm

stdlib/builtins.pyi

@@ -2306,7 +2306,7 @@ class BaseException:
     __suppress_context__: bool
     __traceback__: TracebackType | None
     def __init__(self, *args: object) -> None: ...
-    def __new__(cls, *args: Any, **kwds: Any) -> Self: ...
+    def __new__(cls, /, *args: Any, **kwds: Any) -> Self: ...
     def __setstate__(self, state: dict[str, Any] | None, /) -> None: ...
     def with_traceback(self, tb: TracebackType | None, /) -> Self: ...
     # Necessary for security-focused static analyzers (e.g, pysa)