input: Write a function to convert all characters of a given string to lowercase. If the input string is empty, return an empty string.
  assert to_lower("HELLO")=="hello"
  assert to_lower("WORLD")=="world"
  assert to_lower("Python3")=="python3"
  assert to_lower("THIS IS A TEST")=="this is a test"
  assert to_lower("")==""
output: def to_lower(s):
      return s.lower()