interfaces: convert `repository.peer` to a Protocol class This is similar to the transformation in 3a90a6fd710d for dirstate. However, `ipeerbase` isn't referenced anywhere other than the `implementer` decorator on this class. I suspect, based on the lack of `self` args on these classes prior to c1674551c109, that zope enforced that interfaces and implementations are separate entities. `Protocol` does not, so fold the former interface definition into the class. Per (seemingly undocumented) pytype desire, the `Protocol` subclass comes last in the list.