move requirements in requirements-dev.in to setup.py

[willkg]

In my other projects, I've moved all the dev requirements into setup.py as extras_require. I think we should do that here, too. It changes the developer workflow to something like this:

pip install '.[dev]'

Then we don't need pip-tools (which isn't listed as a dependency anywhere).

[g-k]

Sound good!

We should update any dev setup and the release process docs to use that package name and call out the package as project internal / unstable API. Version-wise I assume it'd match the non-dev bleach version

[willkg]

This is fixed in #640.