Use PEP 570 syntax in stdlib (#11250)

stdlib/email/header.pyi

@@ -17,7 +17,7 @@ class Header:
     def append(self, s: bytes | bytearray | str, charset: Charset | str | None = None, errors: str = "strict") -> None: ...
     def encode(self, splitchars: str = ";, \t", maxlinelen: int | None = None, linesep: str = "\n") -> str: ...
     def __eq__(self, other: object) -> bool: ...
-    def __ne__(self, __value: object) -> bool: ...
+    def __ne__(self, value: object, /) -> bool: ...
 
 # decode_header() either returns list[tuple[str, None]] if the header
 # contains no encoded parts, or list[tuple[bytes, str | None]] if the header