bpo-35404: Clarify how to import _structure in email.message doc

[charlax]

The example for walk() in email.message.EmailMessage makes use of the _structure function but does not clarify how to import it.

https://bugs.python.org/issue35404

[the-knights-who-say-ni]

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[freddrake]

LGTM

[charlax]

Thanks @briancurtin!