input: Write a python function to calculate the sum of each row in a 2D list.
  assert row_sums([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [6, 15, 24]
  assert row_sums([[0, 0, 0], [1, 2, 3], [4, 5, 6]]) == [0, 6, 15]
  assert row_sums([[1], [2], [3]]) == [1, 2, 3]
output: def row_sums(matrix):
      return [sum(row) for row in matrix]