input: Write a python function to check if a string is an isogram.
  assert is_isogram("isogram") == True
  assert is_isogram("eleven") == False
  assert is_isogram("subdermatoglyphic") == True
  assert is_isogram("") == True
  assert is_isogram("Alphabet") == False
output: def is_isogram(s):
      s = s.lower()
      return len(s) == len(set(s))