gh-100991: fix email header References and In-Reply-To being treated as unstructured

[RanKKI]

According to RFC 5322#3.6.4, both References and In-Reply-To are not unstructured fields and should have similar behaviour to Message-ID.

message-id      =   "Message-ID:" msg-id CRLF
in-reply-to     =   "In-Reply-To:" 1*msg-id CRLF
references      =   "References:" 1*msg-id CRLF

---

Sample Code

msg = textwrap.dedent(f"""\
            Message-ID: <long-but-perfectly-valid-message-id-that-does-not-end-up-qp-encoded@example.com>
            In-Reply-To: <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com>
            References: <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com> <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com>
            """)
msg = email.message_from_string(msg, policy=email.policy.default)

Before this patch

Message-ID: <long-but-perfectly-valid-message-id-that-does-not-end-up-qp-encoded@example.com>
In-Reply-To: =?utf-8?q?=3Creference-to-long-but-perfectly-valid-message-id-t?=
 =?utf-8?q?hat-gets-qp-encoded=40example=2Ecom=3E?=
References: =?utf-8?q?=3Creference-to-long-but-perfectly-valid-message-id-th?=
 =?utf-8?q?at-gets-qp-encoded=40example=2Ecom=3E_=3Creference-to-long-but-pe?=
 =?utf-8?q?rfectly-valid-message-id-that-gets-qp-encoded=40example=2Ecom=3E?=

After this patch

Message-ID: <long-but-perfectly-valid-message-id-that-does-not-end-up-qp-encoded@example.com>
In-Reply-To: <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com>
References: <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com>
 <reference-to-long-but-perfectly-valid-message-id-that-gets-qp-encoded@example.com>

• Issue: email module incorrectly qp-encodes References and In-Reply-To headers #100991