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/threading.rst b/Doc/library/threading.rst
index 0b46349..9544466 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -4,6 +4,9 @@
 .. module:: threading
    :synopsis: Higher-level threading interface.
 
+**Source code:** :source:`Lib/threading.py`
+
+--------------
 
 This module constructs higher-level threading interfaces on top of the  lower
 level :mod:`thread` module.
@@ -36,11 +39,6 @@
    :mod:`multiprocessing`. However, threading is still an appropriate model
    if you want to run multiple I/O-bound tasks simultaneously.
 
-.. seealso::
-
-   Latest version of the `threading module Python source code
-   <http://svn.python.org/view/python/branches/release27-maint/Lib/threading.py?view=markup>`_
-
 
 This module defines the following functions and objects: