input: Write a python function to decrypt a Caesar cipher text by shifting each letter back by 3 positions.
  assert caeser_encryption_decrypt("xlmw mw xli pefsi") == "text it the laser"
  assert caeser_encryption_decrypt("khoor zruog") == "hello world"
  assert caeser_encryption_decrypt("dwwdfn dw gkhg") == "attack at dawn"
output: def caeser_encryption_decrypt(cipher_text):
      decrypted_text = ""
      for char in cipher_text:
          if char.isalpha():
              shift = ord('a') if char.islower() else ord('A')
              decrypted_text += chr((ord(char) - shift - 3) % 26 + shift)
          else:
              decrypted_text += char
      return decrypted_text