input: Write a python function to find the first non-repeating letter in a string.
  assert first_non_repeating_letter("apple") == "a"
  assert first_non_repeating_letter("stress") == "t"
  assert first_non_repeating_letter("moonmen") == "e"
output: def first_non_repeating_letter(s):
      for char in s:
          if s.lower().count(char.lower()) == 1:
              return char
      return ""