update to new MutableMapping import path

collections.MutableMapping is deprecated since version Python 3.3
and was removed in Python 3.10

admin/doorman/storage/classes.py

@@ -1,5 +1,5 @@
 from sys import stderr
-from collections import MutableMapping
+from collections.abc import MutableMapping
 from io import StringIO
 
 import csv, json
@@ -52,4 +52,3 @@ class JsonStorage(Storage):
         return stored
     def encode(self, data):
         return json.dumps(data, indent=4)
-