Backport source links from 3.x.
Existing links have been updated to use the new reST role. In some
files, I have also made cosmetic changes to the header.
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index 9c65587..7929c51 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -11,6 +11,10 @@
.. versionadded:: 2.1
+**Source code:** :source:`Lib/weakref.py`
+
+--------------
+
The :mod:`weakref` module allows the Python programmer to create :dfn:`weak
references` to objects.