Create version 4.0

[alarthast]

Part of #224 .

Version 4 removes support for cohort extractor actions. It therefore no longer mandates an expectations configuration in the project.yaml file, and raises a ValidationError if the file includes a cohort extractor action.

The expectations attribute of the Pipeline class for v4 is now None rather than an Expectations class. This is new, so the v4 pipeline release needs to be tested before we switch the research-template to using it.

[evansd]

I really appreciated the logical sequencing of the commits here. It made it very easy to review.

If you wanted to test this in action with a real project without having to first dance around updating a chain of four different packages, there is a way to do so.

The opensafely run command is just a wrapper around a command in job-runner called local_run. So you could do something like this:

1. Get job-runner checked out and working locally.

2. Install your version of the pipeline library directly from this branch:

   pip install pip install opensafely-pipeline@git+https://github.com/opensafely-core/pipeline@create-v4
   

3. Run:

   python -m jobrunner.cli.local_run --project-dir <path/to/some/project>
   

[alarthast]

Followed the above instructions and locally things are as expected.