fix issues with 'Extractor.finalize()'

- prevent crash in InstagramUserExtractor (#4359)
- call it at the end of every DownloadJob
- add it to tests

gallery_dl/extractor/common.py

@@ -36,7 +36,6 @@ class Extractor():
     browser = None
     root = ""
     test = None
-    finalize = None
     request_interval = 0.0
     request_interval_min = 0.0
     request_timestamp = 0.0
@@ -66,6 +65,9 @@ class Extractor():
         self._init()
         self.initialize = util.noop
 
+    def finalize(self):
+        pass
+
     def items(self):
         yield Message.Version, 1