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/atexit.rst b/Doc/library/atexit.rst
index fd884d7..c70dcf2 100644
--- a/Doc/library/atexit.rst
+++ b/Doc/library/atexit.rst
@@ -1,4 +1,3 @@
-
 :mod:`atexit` --- Exit handlers
 ===============================
 
@@ -10,17 +9,16 @@
 
 .. versionadded:: 2.0
 
+**Source code:** :source:`Lib/atexit.py`
+
+--------------
+
 The :mod:`atexit` module defines a single function to register cleanup
 functions.  Functions thus registered are automatically executed upon normal
 interpreter termination.  The order in which the functions are called is not
 defined; if you have cleanup operations that depend on each other, you should
 wrap them in a function and register that one.  This keeps :mod:`atexit` simple.
 
-.. seealso::
-
-   Latest version of the `atexit Python source code
-   <http://svn.python.org/view/python/branches/release27-maint/Lib/atexit.py?view=markup>`_
-
 Note: the functions registered via this module are not called when the program
 is killed by a signal not handled by Python, when a Python fatal internal error
 is detected, or when :func:`os._exit` is called.