Preserve MIMEPart header param in returns

Commit 56db30e made Message generic and
switched get_body() and iter_parts() to return
MIMEPart[_HeaderRegistryT].

That dropped the second type parameter, so the header parameter type
defaulted to Any. Preserve both type parameters in the return types and
add a regression test covering get_body() and iter_parts().

Author: tamird

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

@@ -1,5 +1,5 @@
 from email.headerregistry import Address, BaseHeader
-from email.message import EmailMessage
+from email.message import EmailMessage, MIMEPart
 from typing_extensions import assert_type
 
 msg = EmailMessage()
@@ -11,5 +11,8 @@
 
 generic_msg: EmailMessage[BaseHeader, str] = EmailMessage()
 assert_type(generic_msg.get("To"), BaseHeader | None)
+assert_type(generic_msg.get_body(), MIMEPart[BaseHeader, str] | None)
 for a in generic_msg.iter_attachments():
     assert_type(a, EmailMessage[BaseHeader, str])
+for p in generic_msg.iter_parts():
+    assert_type(p, MIMEPart[BaseHeader, str])

stdlib/email/message.pyi

@@ -151,13 +151,15 @@ class Message(Generic[_HeaderT_co, _HeaderParamT_contra]):
 
 class MIMEPart(Message[_HeaderRegistryT_co, _HeaderRegistryParamT_contra]):
     def __init__(self, policy: Policy[Any] | None = None) -> None: ...
-    def get_body(self, preferencelist: Sequence[str] = ("related", "html", "plain")) -> MIMEPart[_HeaderRegistryT_co] | None: ...
+    def get_body(
+        self, preferencelist: Sequence[str] = ("related", "html", "plain")
+    ) -> MIMEPart[_HeaderRegistryT_co, _HeaderRegistryParamT_contra] | None: ...
     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_co]]: ...
+    def iter_parts(self) -> Iterator[MIMEPart[_HeaderRegistryT_co, _HeaderRegistryParamT_contra]]: ...
     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: ...
     def make_related(self, boundary: str | None = None) -> None: ...