input: Write a function to reverse an input string. The function should take a string as input and return the reversed string.
  def rev_string(s):
     return s[::-1]
  
  assert rev_string("Hello World!") == "!dlroW olleH"
  assert rev_string("Python") == "nohtyP"
  assert rev_string("12345") == "54321"
  assert rev_string("") == ""
  assert rev_string("a") == "a"
output: def rev_string(s):
     return s[::-1]