Drop support for Python 3.6, enable support for Markdown 3.4

[tomaszn]

There are incompatibilities with Markdown 3.4.

@agusmakmun Can you take a look at these? https://github.com/Python-Markdown/markdown/blob/master/docs/change_log/release-3.4.md

I suggest releasing this as v1.7.0 (bump the minor version).