Async generators for python 3.5

[maartenbreddels]

This packages provides async generators for Python3.5:
https://pypi.org/project/async_generator/

Is there interest in using this? Let's say if I open a PR, would that make a change of being merged?

[ecederstrand]

Either that, or python_requires in setup.py should be changed to not advertise Python 3.5 support. The jinja2.asyncfilters and jinja2.asyncsupport contain syntax errors when imported in Python 3.5.

[davidism]

Jinja does support Python 3.5. Async support is Python 3.6+. While the library is interesting, I don't want to add a required external dependency.