I had PYTYHONPYCACHEPREFIX=/tmp/pyc in my local shell, which put pyc
files elsewhere.  A few tests care very much about .pyc files, so those
tests failed.  This setting in tox.ini will ensure that .pyc files are
written locally so the tests will pass.

Coverage.py predates sets as a built-in data structure, so the file data
collection has long been dicts with None as the values.  Sets are
available to us now (since Python 2.4 in 2004, which coverage.py dropped
support for in 2014!), we use sets.

The comprehension would only completely finish (->exit) if the assert
was going to fail.  So we expect it to not finish.

The old pycache_prefix logic was because we would run tests in Docker
containers, and they went faster if we set PYTHONCACHEPREFIX.  But we
don't do that anymore, so we can use simpler code.