setup: bump celery to version ~4.0

[jacquerie]

Description:

It should be noted that hepcrawl has been tested against version 4 of Celery since that version was released due to how the constraint is expressed, so nothing is expected to break here.

Checklist:

• [x] I have all the information that I need (if not, move to RFC and look for it).
• [ ] I linked the related issue(s) in the corresponding commit logs.
• [x] I wrote good commit log messages.
• [x] My code follows the code style of this project.
• [ ] I've added any new docs if API/utils methods were added.
• [ ] I have updated the existing documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[david-caro]

Though I don't understand why hepcrawl needs celery in the first place. The whole hepcrawl/inspire-crawler/workflow interaction is very confusing.

So it needs celery to be able to trigger a task, the flow is: hepcrawl -> celery task -> inspire-crawler -> workflow

inspire-crawler is the part on inspire-next that receives the crawl results and generates workflows from them. Celery is just the protocol used between hepcrawl and inspire-crawler.