Update embedded jinja2

[thirtytwobits]

It appears we are on borrowed time with the package loader in jinja relying on a deprecated API. The best way forward is to pull in the latest subtree.

[thirtytwobits]

Removing major label since we did a partial update in 2.1.0 and not much broke. This suggests we're not so far off the mainline that this can't wait a few more months or so.