input: Write a python function to convert a list of characters into a single concatenated string.
  assert list_to_str(['a', 'b', 'c']) == 'abc'
  assert list_to_str(['h', 'e', 'l', 'l', 'o']) == 'hello'
  assert list_to_str(['P', 'y', 't', 'h', 'o', 'n']) == 'Python'
output: def list_to_str(char_list):
      return ''.join(char_list)