[docs] [issue35404] Document how to import _structure in email.message

[Roundup Robot]

Change by Roundup Robot <devnull at psf.upfronthosting.co.za>:


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keywords: +patch
pull_requests: +10124
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35404>
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