Any reason why Django wasn't tested? It supports both the sync standard and async stanadard and is AFAIK the most popular web framework (way more then flask)
Django is not async yet. You can run it over ASGI but parts of it (eg ORM) need extra compat layer (sync_to_async wrapper) for async to work.
Still, should have been tested as a sync framework alongside flask
Django 3 is only async at the view layer. So if you just want to return a static "Hello World" string then you'll be async. But if you do any I/O then it will block.