Make the JSONValue type recursive

[ewjoachim]

Relates to #1385 : we discovered the Json type isn't recursive even though it should be.

Successful PR Checklist:

• Tests
  • (not applicable?)
• Documentation
  • (not applicable?)

PR label(s):

• PR type: breaking 💥 Contains breaking changes
• PR type: feature ⭐️ Contains new features
• PR type: bugfix 🕵️ Contains bug fix
• PR type: miscellaneous 👾 Contains misc changes
• PR type: dependencies 🤖 Contains only dependencies updates
• PR type: documentation 📚 Contains documentation updates

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate
types.py
worker.py
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