mailbox: Couple small fixes (#3490)

* mailbox: Make stub-only helpers private

These HasIteritems and HasItems protocols don't exist in the real
mailbox module so prefix them with underscore to indicate they are
private.

* mailbox: Fix type of message argument

The Mailbox.add and Mailbox.__setitem__ methods take a message argument
which can be anything that is convertible to a Message.  Fix the
signatures accordingly.

Author: suutari

stdlib/2and3/mailbox.pyi

@@ -19,10 +19,10 @@ _T = TypeVar("_T")
 _MessageType = TypeVar("_MessageType", bound=Message)
 _MessageData = Union[email.message.Message, bytes, str, IO[str], IO[bytes]]
 
-class HasIteritems(Protocol):
+class _HasIteritems(Protocol):
     def iteritems(self) -> Iterator[Tuple[str, Message]]: ...
 
-class HasItems(Protocol):
+class _HasItems(Protocol):
     def items(self) -> Iterator[Tuple[str, Message]]: ...
 
 linesep: bytes
@@ -33,11 +33,11 @@ class Mailbox(Generic[_MessageType]):
     _factory: Optional[Callable[[IO[Any]], _MessageType]]  # undocumented
 
     def __init__(self, path: _PathType, factory: Optional[Callable[[IO[Any]], _MessageType]] = ..., create: bool = ...) -> None: ...
-    def add(self, message: _MessageType) -> str: ...
+    def add(self, message: _MessageData) -> str: ...
     def remove(self, key: str) -> None: ...
     def __delitem__(self, key: str) -> None: ...
     def discard(self, key: str) -> None: ...
-    def __setitem__(self, key: str, message: _MessageType) -> None: ...
+    def __setitem__(self, key: str, message: _MessageData) -> None: ...
     @overload
     def get(self, key: str, default: None = ...) -> Optional[_MessageType]: ...
     @overload
@@ -63,7 +63,7 @@ class Mailbox(Generic[_MessageType]):
     @overload
     def pop(self, key: str, default: _T = ...) -> Union[_MessageType, _T]: ...
     def popitem(self) -> Tuple[str, _MessageType]: ...
-    def update(self, arg: Optional[Union[HasIteritems, HasItems, Iterable[Tuple[str, _MessageType]]]] = ...) -> None: ...
+    def update(self, arg: Optional[Union[_HasIteritems, _HasItems, Iterable[Tuple[str, _MessageType]]]] = ...) -> None: ...
     def flush(self) -> None: ...
     def lock(self) -> None: ...
     def unlock(self) -> None: ...