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/ast.rst b/Doc/library/ast.rst
index d9b2212..5130d00 100644
--- a/Doc/library/ast.rst
+++ b/Doc/library/ast.rst
@@ -13,6 +13,9 @@
 .. versionadded:: 2.6
    The high-level ``ast`` module containing all helpers.
 
+**Source code:** :source:`Lib/ast.py`
+
+--------------
 
 The :mod:`ast` module helps Python applications to process trees of the Python
 abstract syntax grammar.  The abstract syntax itself might change with each
@@ -26,11 +29,6 @@
 compiled into a Python code object using the built-in :func:`compile` function.
 
 
-.. seealso::
-
-   Latest version of the `ast module Python source code
-   <http://svn.python.org/view/python/branches/release27-maint/Lib/ast.py?view=markup>`_
-
 Node classes
 ------------