Add type annotations

nbxmpp/protocol.py

@@ -936,7 +936,7 @@ class Protocol(Node):
         """
         self.setAttr('id', val)
 
-    def getError(self):
+    def getError(self) -> Optional[str]:
         """
         Return the error-condition (if present) or the textual description
         of the error (otherwise)

nbxmpp/task.py

@@ -267,7 +267,7 @@ class Task:
             return self._error
         return self._result
 
-    def finish(self):
+    def finish(self) -> Any:
         if self._error is not None:
             raise self._error  # pylint: disable=raising-bad-type
         return self._result