input: Write a python function to convert a list of words to lowercase.
  assert to_lowercase(["Hello", "WORLD", "PYTHON", "ProgRAMMING"]) == ["hello", "world", "python", "programming"]
  assert to_lowercase(["TEST", "CaSe", "ConVERSION"]) == ["test", "case", "conversion"]
  assert to_lowercase(["MiXeD", "CASE", "SENTENCES"]) == ["mixed", "case", "sentences"]
output: def to_lowercase(words):
      return [word.lower() for word in words]