CI failing due to pymc import

The CI for Python 3.9 is failing because of an incompatibility between the latest scipy release (1.13.0) and arviz, (see e.g. https://git.ligo.org/lscsoft/bilby/-/jobs/3276928). Arviz has already made a release fixing this but it only supports >= Python 3.10, so that 3.9 CI will continue to fail.