Abstract classes should use the abc module

[ryan-collingham]

Testplan is structured to make heavy use of inheritance between abstract base classes and concrete subclasses. To ease the development effort of creating a new subclass, we should leverage the standard library abc module to mark classes as abstract via the ABCMeta metaclass and to mark particular methods as abstract via the abstractmethod decorator.

Not only will this make it clearer visually which classes and methods are abstract, it will also enable a runtime check to ensure that all abstract methods are overridden by subclasses, therefore making it less likely to hit a NotImplementedError later at runtime.