input: Write a function to convert all characters of a given string to lowercase.
  def to_lower(string):
      return string.lower()
  assert to_lower("HELLO")=="hello"
  assert to_lower("WORLD")=="world"
  assert to_lower("")==""
  assert to_lower("Python3.9")=="python3.9"
  assert to_lower("aBcDeFgHiJkLmNoPqRsTuVwXyZ")=="abcdefghijklmnopqrstuvwxyz"
output: def to_lower(string):
      return string.lower()