title: Pydantic enum field does not get converted to string
body: <p>I am trying to restrict one field in a class to an enum. However, when I try to get a dictionary out of class, it doesn't get converted to string. Instead it retains the enum. I checked <a href="https://pydantic-docs.helpmanual.io/usage/types/#enums-and-choices" rel="noreferrer">pydantic documentation</a>, but couldn't find anything relevant to my problem.</p>
  <p>This code is representative of what I actually need.</p>
  <pre><code>from enum import Enum
  from pydantic import BaseModel
  
  class S(str, Enum):
      am='am'
      pm='pm'
  
  class K(BaseModel):
      k:S
      z:str
  
  a = K(k='am', z='rrrr')
  print(a.dict()) # {'k': &lt;S.am: 'am'&gt;, 'z': 'rrrr'}
  </code></pre>
  <p>I'm trying to get the <code>.dict()</code> method to return <code>{'k': 'am', 'z': 'rrrr'}</code></p>