Re-enable test coverage for CI

[phmccarty]

As mentioned in #660 (comment), the coverage tool is no longer present in the python-extras bundle. My best understanding is that coverage was pulled into the bundle originally via runtime dependencies, not via direct listing in the bundle definition. And once that dependency link was severed (from a package (or packages) that I have not identified :-) ), the tool disappeared from the bundle.

It is desirable to re-enable coverage analysis for CI runs. The options I see to resolve are:

1. Add pypi-coverage to python-extras, since the CI container installs that bundle already.
2. Add pypi-coverage to a different bundle if that's more appropriate, and also update the Dockerfile (referenced above) to install that bundle as well.

[phmccarty]

I opted to implement option 1, simply adding pypi-coverage to python-extras.

We can re-enable the coverage scans at some point after the Dockerhub image is updated to 36120 or newer.