extract_toc: Explicitly set the html parser to make sure no extra tags get added.

BeautifulSoup supports multiple html parsers. Some of those parsers
try to make the html valid by adding/removing tags[1]. This can lead
to useless html, head & body tags in the final document. By explicitly
setting the parser to ’html.parser’ this behaviour can be avoided.

[1] http://www.crummy.com/software/BeautifulSoup/bs4/doc/#differences-between-parsers