Backport of weakref.WeakSet and tests from Python 3.
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index 46bfc9d..9c65587 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -209,6 +209,14 @@
.. versionadded:: 2.5
+.. class:: WeakSet([elements])
+
+ Set class that keeps weak references to its elements. An element will be
+ discarded when no strong reference to it exists any more.
+
+ .. versionadded:: 2.7
+
+
.. data:: ReferenceType
The type object for weak references objects.