mailbox: improve bytes handling (#9083)

- The _MessageData alias refers to objects handled in the
  _dump_message method: 016c7d37b6/Lib/mailbox.py (L210)
- The path passed to __init__ should be a str path because there are
  many places in the file (search for self._path) where we join it
  with a str, which won't work with a bytes path.

stdlib/mailbox.pyi

@@ -1,6 +1,7 @@
 import email.message
+import io
 import sys
-from _typeshed import Self, StrOrBytesPath
+from _typeshed import Self, StrPath, SupportsNoArgReadline, SupportsRead
 from abc import ABCMeta, abstractmethod
 from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
 from types import TracebackType
@@ -32,7 +33,10 @@ __all__ = [
 
 _T = TypeVar("_T")
 _MessageT = TypeVar("_MessageT", bound=Message)
-_MessageData: TypeAlias = email.message.Message | bytes | str | IO[str] | IO[bytes]
+
+class _SupportsReadAndReadline(SupportsRead[bytes], SupportsNoArgReadline[bytes], Protocol): ...
+
+_MessageData: TypeAlias = email.message.Message | bytes | str | io.StringIO | _SupportsReadAndReadline
 
 class _HasIteritems(Protocol):
     def iteritems(self) -> Iterator[tuple[str, _MessageData]]: ...
@@ -43,13 +47,12 @@ class _HasItems(Protocol):
 linesep: bytes
 
 class Mailbox(Generic[_MessageT]):
-
-    _path: bytes | str  # undocumented
+    _path: str  # undocumented
     _factory: Callable[[IO[Any]], _MessageT] | None  # undocumented
     @overload
-    def __init__(self, path: StrOrBytesPath, factory: Callable[[IO[Any]], _MessageT], create: bool = ...) -> None: ...
+    def __init__(self, path: StrPath, factory: Callable[[IO[Any]], _MessageT], create: bool = ...) -> None: ...
     @overload
-    def __init__(self, path: StrOrBytesPath, factory: None = ..., create: bool = ...) -> None: ...
+    def __init__(self, path: StrPath, factory: None = ..., create: bool = ...) -> None: ...
     @abstractmethod
     def add(self, message: _MessageData) -> str: ...
     @abstractmethod
@@ -105,7 +108,7 @@ class Maildir(Mailbox[MaildirMessage]):
 
     colon: str
     def __init__(
-        self, dirname: StrOrBytesPath, factory: Callable[[IO[Any]], MaildirMessage] | None = ..., create: bool = ...
+        self, dirname: StrPath, factory: Callable[[IO[Any]], MaildirMessage] | None = ..., create: bool = ...
     ) -> None: ...
     def add(self, message: _MessageData) -> str: ...
     def remove(self, key: str) -> None: ...
@@ -146,19 +149,13 @@ class _mboxMMDF(_singlefileMailbox[_MessageT]):
     def get_string(self, key: str, from_: bool = ...) -> str: ...
 
 class mbox(_mboxMMDF[mboxMessage]):
-    def __init__(
-        self, path: StrOrBytesPath, factory: Callable[[IO[Any]], mboxMessage] | None = ..., create: bool = ...
-    ) -> None: ...
+    def __init__(self, path: StrPath, factory: Callable[[IO[Any]], mboxMessage] | None = ..., create: bool = ...) -> None: ...
 
 class MMDF(_mboxMMDF[MMDFMessage]):
-    def __init__(
-        self, path: StrOrBytesPath, factory: Callable[[IO[Any]], MMDFMessage] | None = ..., create: bool = ...
-    ) -> None: ...
+    def __init__(self, path: StrPath, factory: Callable[[IO[Any]], MMDFMessage] | None = ..., create: bool = ...) -> None: ...
 
 class MH(Mailbox[MHMessage]):
-    def __init__(
-        self, path: StrOrBytesPath, factory: Callable[[IO[Any]], MHMessage] | None = ..., create: bool = ...
-    ) -> None: ...
+    def __init__(self, path: StrPath, factory: Callable[[IO[Any]], MHMessage] | None = ..., create: bool = ...) -> None: ...
     def add(self, message: _MessageData) -> str: ...
     def remove(self, key: str) -> None: ...
     def __setitem__(self, key: str, message: _MessageData) -> None: ...
@@ -173,17 +170,15 @@ class MH(Mailbox[MHMessage]):
     def unlock(self) -> None: ...
     def close(self) -> None: ...
     def list_folders(self) -> list[str]: ...
-    def get_folder(self, folder: StrOrBytesPath) -> MH: ...
-    def add_folder(self, folder: StrOrBytesPath) -> MH: ...
-    def remove_folder(self, folder: StrOrBytesPath) -> None: ...
+    def get_folder(self, folder: StrPath) -> MH: ...
+    def add_folder(self, folder: StrPath) -> MH: ...
+    def remove_folder(self, folder: StrPath) -> None: ...
     def get_sequences(self) -> dict[str, list[int]]: ...
     def set_sequences(self, sequences: Mapping[str, Sequence[int]]) -> None: ...
     def pack(self) -> None: ...
 
 class Babyl(_singlefileMailbox[BabylMessage]):
-    def __init__(
-        self, path: StrOrBytesPath, factory: Callable[[IO[Any]], BabylMessage] | None = ..., create: bool = ...
-    ) -> None: ...
+    def __init__(self, path: StrPath, factory: Callable[[IO[Any]], BabylMessage] | None = ..., create: bool = ...) -> None: ...
     def get_message(self, key: str) -> BabylMessage: ...
     def get_bytes(self, key: str) -> bytes: ...
     def get_file(self, key: str) -> IO[bytes]: ...