The `update` method calls `self[key] = message` for each `(key, message)`
pair so it also accepts all the same message types as the `__setitem__`
method. Fix the signature so that the inputs are not restricted to
`Message` instances or to instance of the `_MessageType` type parameter,
but to the more reluctant `_MessageData` type alias.
* mailbox: Make stub-only helpers private
These HasIteritems and HasItems protocols don't exist in the real
mailbox module so prefix them with underscore to indicate they are
private.
* mailbox: Fix type of message argument
The Mailbox.add and Mailbox.__setitem__ methods take a message argument
which can be anything that is convertible to a Message. Fix the
signatures accordingly.
