Run unit tests on three operating systems

[hugovk]

After removing sed in #272, we should now support Windows in addition to Linux and macOS?

Let's test them to avoid regressions.

[AA-Turner]

This doesn't exercise the sed code. If we want to test on multiple OSes then perhaps prefer adding it to the integration test?

A

[hugovk]

Yeah, the integration test was failing when I opened the PR and fixed a couple of minutes later :) Updated, also added 3.14.

There is also value running the unit tests on the other operating systems, we're able to break code there too, whether it's exercises the sed code or not.

[hugovk]

Integration tests were silently passing even when really failing -- that was fixed in #286 and so they do properly fail when failing now.

And the integration tests were really failing here on Windows ("FileNotFoundError: [WinError 2] The system cannot find the file specified") and macOS ("rsync: --delete-delay: unknown option") and 3.14 ("ERROR: Failed building wheel for pillow"), so for now let's keep this PR just for unit tests after all.