multipart/related mail generated with EmailMessage() class lacks mandatory MIME-Version header

Bug report

Bug description:

When creating an email with EmailMessage() that contains an HTML part with images using a multipart/related structure as the outter MIME structure, the resulting mail contains non MIME-Version header. While the only valid value for the MIME-Version header is "1.0", this is a required header field for MIME structures, see https://datatracker.ietf.org/doc/html/rfc2045#section-4

Here is a simple example code:

#!/usr/bin/python3

import email.message

svgimg='<svg xmlns="http://www.w3.org/2000/svg"><circle cx="100" cy="100" r="50" fill="green"/></svg>'

outer = email.message.EmailMessage()

outer.make_related()

inner = email.message.EmailMessage()
inner.set_content("Hello")
inner.add_alternative("<img src='cid:imgid'>", subtype="html")

outer.attach(inner)
outer.add_related(svgimg.encode(), "image", "svg", cid="imgid")

print(outer.as_string())

The output starts like this:

From: test@example.com
To: test@example.org
Subject: Hello
Content-Type: multipart/related;
 boundary="===============4597207535328563093=="

[...]

It needs a "MIME-Version: 1.0" header in the main part. This will also cause some spam rating in e.g. spamassassin.

(Note: The example code uses multipart/related as the outer structure and multipart/alternative as the inner structure. While counterintuitive, this is how it is usually done by almost all mail clients. The example given in the python docs here https://docs.python.org/3/library/email.examples.html does it the other way round.)

CPython versions tested on:

3.12

Operating systems tested on:

Linux

使用報告中的範例，重現透過 email.message.EmailMessage()、make_related()、attach()、add_related() 和 as_string() 產生的問題。追蹤外層 multipart/related 訊息的序列化過程，然後驗證產生的輸出包含 MIME-Version: 1.0，並保留現有的 MIME 結構。