Set the default value in a saner way, add type annotation.

nixops/diff.py

@@ -59,7 +59,7 @@ def __init__(
         res_type: str,
     ):
         # type: (...) -> None
-        self.handlers = None
+        self.handlers: List[Handler] = []
         self._definition = config
         self._state = state
         self._depl = depl