input: Write a python function to implement a substitution cipher that shifts letters by 13 positions in the alphabet.
  assert substitution_cipher("abc") == "nop"
  assert substitution_cipher("xyz") == "klm"
  assert substitution_cipher("hello") == "uryyb"
output: def substitution_cipher(text):
      def shift_13(c):
          if 'a' <= c <= 'z':
              return chr((ord(c) - ord('a') + 13) % 26 + ord('a'))
          elif 'A' <= c <= 'Z':
              return chr((ord(c) - ord('A') + 13) % 26 + ord('A'))
          else:
              return c
  
      return ''.join(shift_13(c) for c in text)