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Switched TestCase to SimpleTestCase where possible in Django's tests.
Fixed #35638 -- Updated validate_constraints to consider db_default.
Refs #34701 -- Moved UniqueConstraint(nulls_distinct) validation tests.
Refs #33476 -- Reformatted code with Black.
Refs #33476 -- Refactored code to strictly match 88 characters line length.
Fixed #35569 -- Improved wording of invalid ForeignKey error message.