Fixing typing for postprocessors

harvester/postprocess.py

 from abc import ABC, abstractmethod
+from typing import Dict, Type
 
 
 class Postprocessor(ABC):
+    def __init__(self, **kwargs):
+        ...
+
     @abstractmethod
     def postprocess(self, item: dict) -> dict:
         pass
 
 
-POSTPROCESSORS = {
+POSTPROCESSORS: Dict[str, Type[Postprocessor]] = {
 
 }