various improvements

[EvanBldy]

Problem

• Some requirements are outdated.
• Due to a change in names_service.get_full_entity_name (it returns a 3-tuple now), there's a ValueError exception.
• Black.
• For Task.difficulty there's no constraint directly added on Postgres.

Solution

• Upgrade outdated requirements.
• Fix ValueError exceptions for names_service.get_full_entity_name.
• Add constraint in migration file + set the default value to 3 and disable null values.