mailbox: Fix signature of Mailbox.update method (#3493)
mailbox: Fix signature of Mailbox.update method (#3493)

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.
euresti/typeshed