CI is broken

```
pymc_experimental/__init__.py:31: in <module>
    from pymc_experimental.marginal_model import MarginalModel
pymc_experimental/marginal_model.py:10: in <module>
    from pymc.logprob.joint_logprob import factorized_joint_logprob
E   ModuleNotFoundError: No module named 'pymc.logprob.joint_logprob'
```

Looks like a PyMC API change.