Use pyproject.toml instead of setuptools

[karpetrosyan]

Should we use pyproject.toml instead of setuptools?
I believe it is preferable to use pyproject.toml, as we do for other encode projects.

Refs: #281

[tomchristie]

I'm okay with us mirroring whatever we're doing with httpx here.

If you choose to open an issue for this then I'd suggest referencing whichever PR introduced pyproject.toml into httpx so that we've got some good interlinking context.