Make sure the UserDict copies do not share the same underlying

dictionary as the original.  This parallels MvL's change to
Lib/os.py 1.56.

Backport candidate.

Lib/UserDict.py

@@ -22,7 +22,7 @@ class UserDict:
     def clear(self): self.data.clear()
     def copy(self):
         if self.__class__ is UserDict:
-            return UserDict(self.data)
+            return UserDict(self.data.copy())
         import copy
         data = self.data
         try: