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/timeit.rst b/Doc/library/timeit.rst
index 21f9796..7fbe19e 100644
--- a/Doc/library/timeit.rst
+++ b/Doc/library/timeit.rst
@@ -1,4 +1,3 @@
-
 :mod:`timeit` --- Measure execution time of small code snippets
 ===============================================================
 
@@ -12,6 +11,10 @@
    single: Benchmarking
    single: Performance
 
+**Source code:** :source:`Lib/timeit.py`
+
+--------------
+
 This module provides a simple way to time small bits of Python code. It has both
 command line as well as callable interfaces.  It avoids a number of common traps
 for measuring execution times.  See also Tim Peters' introduction to the