Start moving checkers in to captainhook.checkers

The idea here is that we move the "check discovery" (which at the moment only consists of looking at a tuple) in to `captainhook.checkers`, which will mean that the hook doesn't have to be re-installed whenever a new checkers is added.

Only did `python3` as I have to go to a meeting now (and didn't want to spend the effort if you didn't like it).