plugin.core: Less introspective DIRECTIVE setting.

Python 3.6 does not implement __class_getitem__, and because of that
it's not possible to introspect when things like Hook[Transaction]
are called. Sidestep the issue with a more explicit assignment.

conservancy_beancount/plugin/core.py

@@ -69,12 +69,10 @@ class Hook(Generic[Entry], metaclass=abc.ABCMeta):
     @abc.abstractmethod
     def run(self, entry: Entry) -> errormod.Iter: ...
 
-    def __init_subclass__(cls) -> None:
-        # cls.__orig_bases__ comes from the ABCMeta metaclass
-        cls.DIRECTIVE = cls.__orig_bases__[0].__args__[0]  # type:ignore[attr-defined]
 
+class TransactionHook(Hook[Transaction]):
+    DIRECTIVE = Transaction
 
-TransactionHook = Hook[Transaction]
 
 ### HELPER CLASSES