* Use conditional imports of trio and anyio

* Update CHANGELOG

* Fan out loggers

* Updated CHANGELOG

* Enable TCP_NODELAY for all synchronous sockets.

The widely documented poor interaction between the Nagle algorithm and
TCP's delayed ACK feature, when making short successive writes, leads to
unnecessary delays (around 50ms on Linux). This happens in httpcore
whenever a POST request is made, since the headers and body are sent as
two separate calls.

The TCP_NODELAY option can be enabled to disable Nagle's algorithm, and
therefore avoid this delay.

The option is already enabled by default for all asyncio and Trio
sockets. It is also enabled by CPython's http.client module (which
urllib and requests use) and by many high-level HTTP libraries found in
other languages, including libcurl, Java's Netty, Rust's reqwest and
Go's standard library, to name a few.

* Update CHANGELOG.md

---------

Co-authored-by: Tom Christie <tom@tomchristie.com>

* Add `sni_hostname` extension

* Fix linting

* Add 'sni_hostname' to `CHANGELOG.md` and `extensions.md`

* Typo

* Update CHANGELOG.md

* Update docs/extensions.md

* Fix changelog

---------

Co-authored-by: Tom Christie <tom@tomchristie.com>

* Add support for socket options

* Use the built-in socket.create_connection method rather than the duplicated one

* Add socket options support for async backend

* Add socket options support for connect_unix_socket function

* Add socket options for ConnectionPool and HTTProxy classes

* Add `socket_options` to `CHANGELOG.md` and `connection-pools.md`

* Make `socket_options` optional for `HTTPConnection` and `ConnectionPool`

* Add `socket_options` to `ConnectionPool` and \'AsyncConnectionPool\' docstring

* Typo

* Fix linting

* Fix options order

* Typo

* Update httpcore/backends/trio.py

Co-authored-by: Tom Christie <tom@tomchristie.com>

* Update httpcore/backends/sync.py

Co-authored-by: Tom Christie <tom@tomchristie.com>

* Update httpcore/backends/asyncio.py

Co-authored-by: Tom Christie <tom@tomchristie.com>

---------

Co-authored-by: Tom Christie <tom@tomchristie.com>

* Version 0.17.2

* Sorting changes by importance

* Sort by importance, fix typo

---------

Co-authored-by: Tom Christie <tom@tomchristie.com>