Update py-markdown2 to 2.1.0

## python-markdown2 2.1.0

- ["nofollow" extra, issue #74, pull #104] Add `rel="nofollow"` support
  (mostly by https://github.com/cdman):

        $ echo '[link](http://example)' | markdown2 -x nofollow
        <p><a rel="nofollow" href="http://example">link</a></p>

  Limitation: This *can* add a duplicate 'rel' attribute to raw HTML links
  in the input.

(obache)