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/zipfile.rst b/Doc/library/zipfile.rst
index 51240ba..30a0361 100644
--- a/Doc/library/zipfile.rst
+++ b/Doc/library/zipfile.rst
@@ -1,4 +1,3 @@
-
 :mod:`zipfile` --- Work with ZIP archives
 =========================================
 
@@ -9,6 +8,10 @@
 
 .. versionadded:: 1.6
 
+**Source code:** :source:`Lib/zipfile.py`
+
+--------------
+
 The ZIP file format is a common archive and compression standard. This module
 provides tools to create, read, write, append, and list a ZIP file.  Any
 advanced use of this module will require an understanding of the format, as