Use build instead of setup.py

Start transitioning into the PEP517 future of Python packaging by
switching from explicitly calling setup.py to using the build
module. Move install requirements into setup.cfg and update the
install docs to refer to the new location. Get rid of the separate
documentation build requirements file by moving into tox.ini deps
for the docs testenv. While we're there, add some commentary in the
tox.ini about some of the more opaque choices we've made for docs
builds and testing YAML style.