Added
* Starlette now officially supports Python3.9.
* In `StreamingResponse`, allow custom async iterator such as objects from classes implementing `__aiter__`.
* Allow usage of `functools.partial` async handlers in Python versions 3.6 and 3.7.
* Add 418 I'm A Teapot status code.
Changed
* Create tasks from handler coroutines before sending them to `asyncio.wait`.
* Use `format_exception` instead of `format_tb` in `ServerErrorMiddleware`'s `debug` responses.
* Be more lenient with handler arguments when using the `requires` decorator.