* Support Python 3.
[0.29.0.gfm.6]
* Fixed polynomial time complexity DoS vulnerability in autolink extension
[0.29.0.gfm.5]
* Added xmpp: and mailto: support to the autolink extension
[0.29.0.gfm.4]
* Remove `source` from list of HTML blocks
[0.29.0.gfm.3]
* Fixed heap memory corruption vulnerabiliy via integer overflow per https://github.com/github/cmark-gfm/security/advisories/GHSA-mc3g-88wq-6f4x
[0.29.0.gfm.2]
* Fixed issues with footnote rendering when used with the autolinker (121),
and when footnotes are adjacent (139).
* We now allow footnotes to be referenced from inside a footnote definition,
we use the footnote label for the fnref href text when rendering html, and
we insert multiple backrefs when a footnote has been referenced multiple
times (229, 230)
* We added new data- attributes to footnote html rendering to make them
easier to style (234)
[0.29.0.gfm.1]
* Fixed denial of service bug in GFM's table extension
per https://github.com/github/cmark-gfm/security/advisories/GHSA-7gc6-9qr5-hc85