Use Self for email.message attachments (#12530)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Avasam <samuel.06@hotmail.com>

stdlib/@tests/test_cases/email/check_message.py

@@ -1,6 +1,10 @@
 from email.headerregistry import Address
 from email.message import EmailMessage
+from typing_extensions import assert_type
 
 msg = EmailMessage()
 msg["To"] = "receiver@example.com"
 msg["From"] = Address("Sender Name", "sender", "example.com")
+
+for a in msg.iter_attachments():
+    assert_type(a, EmailMessage)

stdlib/email/message.pyi

@@ -147,7 +147,11 @@ class Message(Generic[_HeaderT, _HeaderParamT]):
 class MIMEPart(Message[_HeaderRegistryT, _HeaderRegistryParamT]):
     def __init__(self, policy: Policy | None = None) -> None: ...
     def get_body(self, preferencelist: Sequence[str] = ("related", "html", "plain")) -> MIMEPart[_HeaderRegistryT] | None: ...
-    def iter_attachments(self) -> Iterator[MIMEPart[_HeaderRegistryT]]: ...
+    def attach(self, payload: Self) -> None: ...  # type: ignore[override]
+    # The attachments are created via type(self) in the attach method. It's theoretically
+    # possible to sneak other attachment types into a MIMEPart instance, but could cause
+    # cause unforseen consequences.
+    def iter_attachments(self) -> Iterator[Self]: ...
     def iter_parts(self) -> Iterator[MIMEPart[_HeaderRegistryT]]: ...
     def get_content(self, *args: Any, content_manager: ContentManager | None = None, **kw: Any) -> Any: ...
     def set_content(self, *args: Any, content_manager: ContentManager | None = None, **kw: Any) -> None: ...