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/htmlparser.rst b/Doc/library/htmlparser.rst
index c114bf8..0cdc7ca 100644
--- a/Doc/library/htmlparser.rst
+++ b/Doc/library/htmlparser.rst
@@ -18,6 +18,10 @@
    single: HTML
    single: XHTML
 
+**Source code:** :source:`Lib/HTMLParser.py`
+
+--------------
+
 This module defines a class :class:`HTMLParser` which serves as the basis for
 parsing text files formatted in HTML (HyperText Mark-up Language) and XHTML.
 Unlike the parser in :mod:`htmllib`, this parser is not based on the SGML parser