title: given a date object in python, how do I subtract one day
body: <p>have a variable I've set up</p>
  
  <p><code>now = datetime.datetime.now()</code></p>
  
  <p>I want to be able to set up a variable, <code>yesterday</code>, that just takes <code>now</code> and subtracts one day from it. </p>
  
  <p>I'm sure that this is super-easy, but how do I do this?</p>