django.test.client: add AsyncClient and dependencies

[david-flok]

This seems to work for me but I'm not sure how one generally goes about generating these signatures nor how to validate them.

[sbdchd]

Looks good!

In terms of generating signatures you can use stubgen or manually write them

for validation you can try the version of Django-types with your type checker and go from there or write some statements in one of the test .py files in the project to exercise the types