Change div wrapper to a figure wrapper

The reasoning is that the translation of Markdown into HTML should be as
concise as possible. The semantics of the figure element seem to fit the
embedded nature of OEmbed content moreso than the div element which is
quite generic.

mdx_oembed/inlinepatterns.py

@@ -22,7 +22,7 @@ class OEmbedLinkPattern(Pattern):
         if html is None:
             return None
         else:
-            html = "<div class=\"oembed\">%s</div>" % html
+            html = "<figure class=\"oembed\">%s</figure>" % html
             placeholder = self.markdown.htmlStash.store(html, True)
             return placeholder