Add a "build" job in tox

[pganssle]

I would like to start phasing out my ad-hoc release.py file in favor of a tox environment, which will allow me to declare explicit dependencies and will make things all-around easier.

For the moment, we can use virtualenv's "publish" function as a template that wraps the existing release.py script.

It should declare a dependency on click >= 7.0. If #869 is already done, you also need to declare a dependency on pep517 >= 0.5.0.

I would like to test this in some way as well, so please add a job in Travis (or Azure pipelines if #868 has already been merged) that runs tox -e build and ensures that it populates the specified directory with a .tar.gz file and a .whl file.

To Do:

• Add build environment
• Add test for build environment

[pganssle]

This was done in #876 and tweaked in #971.