Verify that built wheels work before releasing to PyPI

[tartley]

In 2019, I made a quick README change, rebuilt a new wheel version, and uploaded it to PyPI. Unfortunately, the wheel build failed in some way, and I didn't notice, so I ended up uploading a broken wheel for v0.4.2.

In order to prevent this happening, I we should automate (or, failing that, document) a process for testing built wheels, by uploading them to test.pypi.org, and checking we can "pip install" them from there.

Also, can/should I delete 0.4.2 from PyPI? Or mark it as bad in some way?

(A related effort is #278, to run tests against a package
built from top level 'src' directory.)