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/getopt.rst b/Doc/library/getopt.rst
index 4d65c79..fb6820f 100644
--- a/Doc/library/getopt.rst
+++ b/Doc/library/getopt.rst
@@ -1,4 +1,3 @@
-
 :mod:`getopt` --- C-style parser for command line options
 =========================================================
 
@@ -6,6 +5,10 @@
    :synopsis: Portable parser for command line options; support both short and long option
               names.
 
+**Source code:** :source:`Lib/getopt.py`
+
+--------------
+
 .. note::
    The :mod:`getopt` module is a parser for command line options whose API is
    designed to be familiar to users of the C :cfunc:`getopt` function. Users who