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.

added Bug High priority Infrastructure labels

approved this merge request

approved this merge request

merged

mentioned in commit 81fca8e3

changed milestone to %1.1.0