Class Summary could be reimplemented as an actual Abstract Base Class

[mpounsett]

Linting complains about the signatures of several methods in this class (no-self-use), and the issue could be resolved by reimplementing them as @staticmethod methods, but that might be API-breaking if anyone is making use of self references in subclassed implementations. If these can't be @staticmethod then they could be reimplemented as a proper ABC using the @abstractmethod decorator.