dict.py: fix test failure due to deepcopy

bilby/core/prior/dict.py

-import copy
 import json
 import os
 import re
@@ -208,7 +207,8 @@ class PriorDict(dict):
 
     def from_dictionary(self, dictionary):
         mvgkwargs = {}
-        for key, val in copy.deepcopy(dictionary).items():
+        for key in list(dictionary.keys()):
+            val = dictionary[key]
             if isinstance(val, Prior):
                 continue
             elif isinstance(val, (int, float)):