message.pyi: Change HeaderType to Any (#2924)

Fixes #2863

Author: utkarsh2102

stdlib/3/email/message.pyi

@@ -15,7 +15,7 @@ _PayloadType = Union[List[Message], str, bytes]
 _CharsetType = Union[Charset, str, None]
 _ParamsType = Union[str, None, Tuple[str, Optional[str], str]]
 _ParamType = Union[str, Tuple[Optional[str], Optional[str], str]]
-_HeaderType = Union[str, Header]
+_HeaderType = Any
 
 class Message:
     preamble: Optional[str]
@@ -33,7 +33,7 @@ class Message:
     def get_charset(self) -> _CharsetType: ...
     def __len__(self) -> int: ...
     def __contains__(self, name: str) -> bool: ...
-    def __getitem__(self, name: str) -> Optional[_HeaderType]: ...
+    def __getitem__(self, name: str) -> _HeaderType: ...
     def __setitem__(self, name: str, val: _HeaderType) -> None: ...
     def __delitem__(self, name: str) -> None: ...
     def keys(self) -> List[str]: ...