title: Python type hints: typing.Mapping vs. typing.Dict
body: <p>I'm working on a python3 project where we use the <code>typing</code> module type hints throughout.</p>
  
  <p>It seems that we use <code>typing.Dict</code> and <code>typing.Mapping</code> pretty much interchangeably.</p>
  
  <p>Is there a reason to prefer one over the other?</p>