Add support for diffcov

[jaraco]

At least two of the projects maintained under the skeleton have a unique diffcov job, requiring separate maintenance downstream. Let's adopt that behavior in the skeleton in order to consolidate the maintenance and avoid stale dependencies and one-off contribs like python/importlib_metadata#467.

[jaraco]

I explored the possibility of integrating with the default test run or at least the default CI job, but it's a little complicated if I don't want to automatically double the number of test runs across all environments, so for now, I've simply adopted the approach used by importlib_metadata except with Python 3.x (instead of 3.9) and using the latest versions of actions.