keep UserDict an old-style class

Be generous in abc.py to allow this.
diff --git a/Lib/UserDict.py b/Lib/UserDict.py
index bd64f84..bb2218a 100644
--- a/Lib/UserDict.py
+++ b/Lib/UserDict.py
@@ -1,6 +1,6 @@
 """A more or less complete user-defined wrapper around dictionary objects."""
 
-class UserDict(object):
+class UserDict:
     def __init__(self, dict=None, **kwargs):
         self.data = {}
         if dict is not None: