This builds off of the initial idea in [this issue][1] of using the
transport API in `httpx`. This allows for using [custom transports][2].

One issue with this current implementation is that there are few more
[changes coming][3] in 0.18.0 that this will probably want to use. So
maybe it will make sense to finalize this on that version before
releasing it. Want to get some eyes on this sooner.

Resolves #263

[1]: encode/httpx#1264 (comment)
[2]: https://www.python-httpx.org/advanced/#custom-transports
[3]: encode/httpx#1522

This is to avoid calling `get_tracer` every time.

Replaces the span and name callbacks

Simplifies the usage of request/response hooks for consumers with
expected objects

Allows for instrumenting and uninstrumenting existing client instances

Not sure how that happened.

Back to only providing the raw arguments and return values instead of
using httpx Request and Response objects to avoid any additional
overhead or unexpected side effects.

Easier usage in hooks

- Move changelog entry
- Fix license copyright
- Add Python 3.9 classifier