Transport API (#963)

Transport API (#963)

* Deprecate Client arg 'dispatch' and use 'transport'

* Remove line in test from coverage

* Document custom transports

* _dispatch > _transports

Also rename *Dispatch classes to *Transport and added aliases

* Fix linting issues

* Missed one _transports import

* Promote URLLib3Transport to public API

* Remove duplicate arg doc

* Assert that urllib3 is imported to use URLLib3Transport

* `AsyncClient`, not asynchronous `Client`

* Add warning category to warn calls

* Update docs/advanced.md

Co-authored-by: Florimond Manca <florimond.manca@gmail.com>
* Add warn_deprecated utility function

* Amend docs references to dispatch

* Add concrete implementation example

* Clearer transport implementation description

Co-authored-by: Florimond Manca <florimond.manca@gmail.com>