Improve CI configuration

[OriolAbril]

We have multiple runs/workers for different python versions, with all of them running the full test suite and one of them also running the tox -e check command and the minimal tests.

As we integrate more of the things we have in arviz-base (for now) we'll need to improve that setting I think. Also for things that are specific to arviz-stats. Some examples:

• arviz-base also runs tests with nightlies of scientific-python libraries
• arviz-base already uses docstub and should have static type checker runs as part of its CI soon-ish Use static type checker arviz-base#67
• arviz-stats is the most bleeding edge out of the 3 libraries when it comes to numpy+scipy usage. I think it would be very nice to run tests on the oldest supported versions to make sure they are actually supported and we aren't using a new numpy/scipy feature without realizing it.