Class Summary could be reimplemented as an actual Abstract Base Class

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.