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/gzip.rst b/Doc/library/gzip.rst
index 037909b..e074bfc 100644
--- a/Doc/library/gzip.rst
+++ b/Doc/library/gzip.rst
@@ -4,6 +4,10 @@
 .. module:: gzip
    :synopsis: Interfaces for gzip compression and decompression using file objects.
 
+**Source code:** :source:`Lib/gzip.py`
+
+--------------
+
 This module provides a simple interface to compress and decompress files just
 like the GNU programs :program:`gzip` and :program:`gunzip` would.