Bump our maximum Markdown version up to 2.6.11
Bump our maximum Markdown version up to 2.6.11

The next commit will fix the warnings, and we need a newer version for
Python 3.9 support because 2.4.1 doesn't work with it.

Markdown 2.6.11 was the last release before 3.0.

Updates #257

veekun/pokedex,veekun/pokedex