input: Write a python function to encrypt a string using a substitution cipher that shifts each character by 3 positions.
  assert encrypt_substitution("abc") == "def"
  assert encrypt_substitution("xyz") == "{|}"
  assert encrypt_substitution("Hello!") == "Khoor$"
output: def encrypt_substitution(s):
      encrypted = ''
      for char in s:
          encrypted += chr(ord(char) + 3)
      return encrypted