title: How do I define a datetime field in a python dataclass?
body: <p>trying to get the syntax of the Python 3.7 new dataclass right.</p>
  
  <p>if I want to include a datetime value in my dataclass,</p>
  
  <pre><code>import datetime
  from dataclasses import dataclass
  
  @dataclass
  class MyExampleWithDateTime:
      mystring: str
      myint: int
      mydatetime: ???
  </code></pre>
  
  <p>What should I write for ??? for a datetime field?</p>