input: Write a python function to normalize a given list of strings.
  assert normalize(['Red', 'red', 'WhITE', 'white', 'bLUE', 'blue']) == ['red', 'red', 'white', 'white', 'blue', 'blue']
  assert normalize(['Hello', 'WORLD', 'TesT']) == ['hello', 'world', 'test']
  assert normalize(['PYTHON', 'Python', 'python']) == ['python', 'python', 'python']
output: def normalize(lst):
      return [s.lower() for s in lst]