Use `pyproject.toml` instead of `setup.cfg` and `setup.py`

[karpetrosyan]

Discussed in: #748

Because httpx uses pyproject.toml for packaging, we should also use pyproject.toml for simplicity.
It would also be preferable to move all project configurations into pyproject toml, as httpx does.

It appears that we will need two PRs to make it much cleaner and more reviewable, one for packaging and one for configurations.

Packaging part

The PR should look very similar to this, so that we can easily migrate based on that PR.

Configuration part

The configuration part is also simple; according to this, we can migrate quickly.

[tomchristie]

Closed via #752