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