Fix ci

This has a few fixes to make the CI run.

• There is a problem with a version of astropy (see https://github.com/astropy/astropy/issues/11093). I've temporarily fixed the version of the dependency that is failing in the ci yml file. This should be removable in the future.
• It looks like pymc3 is pretty broken for python 3.6. There's a problem between the latest version of arviz and pymc3 available as one of the two forgot to make the release explicitly not 3.6 compatible.
• I also added a catch for when pypolychord isn't installed properly the tests won't fall over.