Switch from askama to rinja

[GuillaumeGomez]

Recently, me and another askama maintainer forked the project into rinja. For more details about the why, I wrote a blog post here. But in short: rinja is ahead of askama now in a lot of areas so I think going forward, your project will benefit more from it.

If you're not interested about this switch, don't hesitate to close the PR.

If you want more information about rinja, don't hesitate to ask!