This change replaces all of the requirements.in files with entries in the
pyproject.toml files. The setup.py file has been left in the project with
a default invoker, which will make it remain usable with `python setup.py <cmd>`
usage. A new tox env has been added which can update the requirements.txt
file (still using pip-compile under the hood). The script will examine the
extras from the pyproject.toml file and feed each of them individually to
pip-compile, such that the comments in the requirements.txt show us which
of the extras is responsible for adding the dependency. The extras included
in the requirements.txt output are found in the tox.ini file.