input: Write a python function to decrypt a given ciphertext by shifting each letter 3 positions backwards in the alphabet.
  assert decrypt("NPKUO") == "KMLRL"
  assert decrypt("XYZ") == "UVW"
  assert decrypt("ABC") == "XYZ"
output: def decrypt(ciphertext):
      decrypted_text = ""
      for char in ciphertext:
          if char.isalpha():
              shifted_char = chr((ord(char) - ord('A') - 3) % 26 + ord('A'))
              decrypted_text += shifted_char
          else:
              decrypted_text += char
      return decrypted_text