The format the test uses is:
Sudoku(data).is_valid()
Your function returns a boolean, so the equivalent to this ends up happening:
result = Sudoku(data) # You return a boolean
if result.is_valid(): ... # AttributeError

If you are dead set on using a function, you need to make sure it returns some kind of object with a method called is_valid() that returns the result. Hint: try using collections.namedtuple