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/userdict.rst b/Doc/library/userdict.rst
index ab9ab04..1c75674 100644
--- a/Doc/library/userdict.rst
+++ b/Doc/library/userdict.rst
@@ -1,4 +1,3 @@
-
 :mod:`UserDict` --- Class wrapper for dictionary objects
 ========================================================
 
@@ -6,6 +5,10 @@
    :synopsis: Class wrapper for dictionary objects.
 
 
+**Source code:** :source:`Lib/UserDict.py`
+
+--------------
+
 The module defines a mixin,  :class:`DictMixin`, defining all dictionary methods
 for classes that already have a minimum mapping interface.  This greatly
 simplifies writing classes that need to be substitutable for dictionaries (such
@@ -19,11 +22,6 @@
 sub-classes that obtained new behaviors by overriding existing methods or adding
 new ones.
 
-.. seealso::
-
-   Latest version of the `UserDict Python source code
-   <http://svn.python.org/view/python/branches/release27-maint/Lib/UserDict.py?view=markup>`_
-
 The :mod:`UserDict` module defines the :class:`UserDict` class and
 :class:`DictMixin`: