title: Using python's mock to temporarily delete an object from a dict
body: <p>I am writing a test for some code that checks for a value in <code>os.environ</code> (I know this isn't optimal, but I have to go with it).  I would like to remove an entry from os.environ for the duration of the test.  I am not sure if mock supports this.  I know <code>patch.dict</code> can be used to modify an item, but I want the key/value pair removed.  I would like something along these lines:</p>
  
  <pre><code>print os.environ
  { ... , 'MY_THING': 'foo', ... }
  
  with mock.patch.dict.delete('os.environ', 'MY_THING'):
      # run the test
      # ( 'MY_THING' in os.environ ) should return False
  
  # everything back to normal now    
  print os.environ
  { ... , 'MY_THING': 'foo', ... }
  </code></pre>
  
  <p>Is there a way to perform such a feat?</p>