CI configuration to run the tests of PySpark against development version of cloudpickle

In the recently merged #236, @pierreglaser added the ability to launch the tests of some downstream projects (loky, joblib and dask/distributed) in order to be able to detect whether if a specific PR is likely to introduce a regression in downstream project just by adding the `[ci downstream]` flag in a commit message of the last commit of the PR.

It would be great to add the build configuration to launch the PySpark test suite in a similar manner.