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/tempfile.rst b/Doc/library/tempfile.rst
index 7ef8a2f..936f06a 100644
--- a/Doc/library/tempfile.rst
+++ b/Doc/library/tempfile.rst
@@ -1,4 +1,3 @@
-
 :mod:`tempfile` --- Generate temporary files and directories
 ============================================================
 
@@ -13,6 +12,10 @@
    pair: temporary; file name
    pair: temporary; file
 
+**Source code:** :source:`Lib/tempfile.py`
+
+--------------
+
 This module generates temporary files and directories.  It works on all
 supported platforms.