Use Self for email.message attachments (#12530)

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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)