Changed
* Preserve HTTP header casing. (Pull 1338, encode/httpcore216, python-hyper/h11104)
* Drop `response.next()` and `response.anext()` methods in favour of `response.next_request` attribute. (Pull 1339)
* Closed clients now raise a runtime error if attempting to send a request. (Pull 1346)
Added
* Add Python 3.9 to officially supported versions.
* Type annotate `__enter__`/`__exit__`/`__aenter__`/`__aexit__` in a way that supports subclasses of `Client` and `AsyncClient`. (Pull 1336)