Moved WeakSet into a bootstap module use by abc.py.
This makes it possible to use ABCs in weakref.py
(which will be done in a later checkin).
diff --git a/Lib/abc.py b/Lib/abc.py
index 8751153..8fb61c6 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -3,7 +3,7 @@
 
 """Abstract Base Classes (ABCs) according to PEP 3119."""
 
-from weakref import WeakSet
+from _weakrefset import WeakSet
 
 def abstractmethod(funcobj):
     """A decorator indicating abstract methods.