`kilter.protocol` 0.4.0 provides a new member for this.

The name of the produced package is correct and should be left alone.

This will be of use in an upcoming change…

This removes some of the boilerplate associated with MockMessageStream

This will stop filters without stopping the entire runner.

This should make no difference to current code, but may help defend
against future breakages.

Closes #10

Closes #11

Closes #9

… but use ~=3.1 for typing, as there are a few typing-related issues in
the 3.0 releases.