gh-147957: pop items from UserDict in LIFO order

`UserDict.popitem` used to pop first-in, first-out since that's the
`MutableMapping` implementation. It now pops first-in, last-out since
that's the behavior guaranteed for `dict` starting in Python 3.7.

Author: Andrew5057

Lib/collections/__init__.py

@@ -1253,6 +1253,13 @@ def copy(self):
         c.update(self)
         return c
 
+
+    # This method has a default implementation in MutableMapping, but dict's
+    # equivalent is first-in, first-out.
+    def popitem(self):
+        return self.data.popitem()
+
+
     @classmethod
     def fromkeys(cls, iterable, value=None):
         d = cls()