This updates our Markdown extensions and testing to require and work

with Python-Markdown 3.x. At the time of this writing, 3.3.3 is the

latest version of this package, and 3.1.1 is the last version to support

Python 2.7. Because of this, we currently need to maintain support

across the 3.x range.
The extensions have been updated for the requirements in Python-Markdown

3.0+, which basically amounts to removing support for the outdated

"safe HTML" modes and switching to some modern functions for

registration of text processors.
Starting in Djblets 3.0, we'll remove support for Markdown 3.1, as we'll

be fully on Python 3.x.

Unit tests pass on Python 2.7 and 3.x, with Markdown 3.1.1 and 3.3.3.