Continuous integration with Traits 6.0

[kitchoi]

Similar to enthought/traitsui#1054

By setting up a CI job against Traits 6.0, we make sure we don't accidentally force projects to upgrade due to dependencies on existing features.

New features can depend on Traits 6.1+ and we can skip tests requiring it in the job with Traits 6.0, i.e. essentially treating Traits 6.1 as an optional dependency. Then downstream projects will have breathing space to opt-in rather than being forced to upgrade.

[kitchoi]

There are quite a few pending PRs that introduce dependencies on Traits 6.1: Imports will fail because those modules import traits.api import observe.
There are already code on master that have this problem, so this will need to be dealt with anyway. We just need to coordinate the PRs a bit. I will see what I can do.