title: Can Python "os.environ.get" ever return a non-string?
body: <p>In this code <a href="https://github.com/chimpler/pyhocon/blob/master/pyhocon/config_parser.py#L291" rel="noreferrer">here</a>, they use <code>os.environ</code> to get the value of an environment variable, and then immediately check to see if it is an instance of their custom classes.</p>
  
  <pre><code>value = os.environ.get(variable)
  ...
  elif isinstance(value, ConfigList) or isinstance(value, ConfigTree):
  </code></pre>
  
  <p>Is it actually possible that the <code>value</code> will be an instance of their custom classes? Is this dead code?</p>