input: Write a python function to swap two variables.
  assert swap_variables(3, 5) == (5, 3)
  assert swap_variables('hello', 'world') == ('world', 'hello')
  assert swap_variables([1, 2, 3], [4, 5, 6]) == ([4, 5, 6], [1, 2, 3])
output: def swap_variables(a, b):
      return b, a