More thorough testing of downstream projects

traitlets underpins lots of Jupyter projects that interact with traitlets in a variety of ways. Traitlets is also extremely complicated and brittle, where small, apparently innocuous (especially cleanup and refactor changes). One of the criteria of traitlets is that changes to it will never ever break a Jupyter application. Many changes during the 5.0 lifecycle have broken the tests in Jupyter packages, and these weren't caught because they aren't tested here.

We should be running the tests of several of these packages as part of the test suite of traitlets itself, and possibly some additional examples of loading various configuration files, to get better confidence that bugs aren't being introduced that affect downstream projects.