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/httplib.rst b/Doc/library/httplib.rst
index 2789c08..e1e3b2a 100644
--- a/Doc/library/httplib.rst
+++ b/Doc/library/httplib.rst
@@ -16,6 +16,10 @@
 
 .. index:: module: urllib
 
+**Source code:** :source:`Lib/httplib.py`
+
+--------------
+
 This module defines classes which implement the client side of the HTTP and
 HTTPS protocols.  It is normally not used directly --- the module :mod:`urllib`
 uses it to handle URLs that use HTTP and HTTPS.