Http3

Added

* Add `httpx.MockTransport()`, allowing to mock out a transport using pre-determined responses. (Pull 1401, Pull 1449)
* Add `httpx.HTTPTransport()` and `httpx.AsyncHTTPTransport()` default transports. (Pull 1399)
* Add mount API support, using `httpx.Client(mounts=...)`. (Pull 1362)
* Add `chunk_size` parameter to `iter_raw()`, `iter_bytes()`, `iter_text()`. (Pull 1277)
* Add `keepalive_expiry` parameter to `httpx.Limits()` configuration. (Pull 1398)
* Add repr to `httpx.Cookies` to display available cookies. (Pull 1411)
* Add support for `params=<tuple>` (previously only `params=<list>` was supported). (Pull 1426)

Fixed

* Add missing `raw_path` to ASGI scope. (Pull 1357)
* Tweak `create_ssl_context` defaults to use `trust_env=True`. (Pull 1447)
* Properly URL-escape WSGI `PATH_INFO`. (Pull 1391)
* Properly set default ports in WSGI transport. (Pull 1469)
* Properly encode slashes when using `base_url`. (Pull 1407)
* Properly map exceptions in `request.aclose()`. (Pull 1465)

Fixed

* Support literal IPv6 addresses in URLs. (Pull 1349)
* Force lowercase headers in ASGI scope dictionaries. (Pull 1351)

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)

Added

* Add `response.next_request` (Pull 1334)

Added

* Support direct comparisons between `Headers` and dicts or lists of two-tuples. Eg. `assert response.headers == {"Content-Length": 24}` (Pull 1326)

Fixed

* Fix automatic `.read()` when `Response` instances are created with `content=<str>` (Pull 1324)

Fixed

* Fixed connection leak in async client due to improper closing of response streams. (Pull 1316)