Experiment: run Jinja in async mode

[simonw]

See http://jinja.pocoo.org/docs/2.10/api/#async-support

Tests all pass. Have not checked performance difference yet.

Creating pull request to run tests in Travis. This is not ready to merge - I'm not yet sure if this is a good idea.

[simonw]

This failed in Python 3.5:

  File "/home/travis/virtualenv/python3.5.6/lib/python3.5/site-packages/jinja2/environment.py", line 1020, in render_async
    raise NotImplementedError('This feature is not available for this '
NotImplementedError: This feature is not available for this version of Python

It looks like this is caused by this feature detection code:

https://github.com/pallets/jinja/blob/a7ba0b637805c53d442e975e3864d3ea38d8743f/jinja2/utils.py#L633-L638

[simonw]

#622 will drop 3.5 support.

[simonw]

Closing this because it was written against Sanic. #628 tracks future work on this topic.