* refactor: derive `AnymailInboundMessage` from `email.message.EmailMessage`
rather than legacy Python 2.7 `email.message.Message`
* feat(inbound): replace confusing `inline_attachments` with `content_id_map`
and `inlines`; rename `is_inline_attachment` to `is_inline`; deprecate old names
Closes#328
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Co-authored-by: Mike Edmunds <medmunds@gmail.com>