Add __all__ for modules beginning with 'h' and 'i' (#7327)

stdlib/imghdr.pyi

@@ -1,6 +1,8 @@
 from _typeshed import StrPath
 from typing import Any, BinaryIO, Callable, Protocol, overload
 
+__all__ = ["what"]
+
 class _ReadableBinary(Protocol):
     def tell(self) -> int: ...
     def read(self, size: int) -> bytes: ...