title: marshmallow - How can I map the schema attribute to another key when serializing?
body: <p>I need to create a marshmallow Schema that is consistent with a given specification, in which I can not alter the key names. One key is the reserved keyword "from" in Python. </p>
  
  <pre><code>class TemporalExtentSchema(Schema):
      from = fields.String(required=True)
      to = fields.String(required=True)
  </code></pre>
  
  <p>This is of course not allowed in Python, so I need to write something like this:</p>
  
  <pre><code>class TemporalExtentSchema(Schema):
      t_from = fields.String(required=True)
      to = fields.String(required=True)
  </code></pre>
  
  <p>What I would like to have is:</p>
  
  <pre><code>{
      "from": "2018-01-01",
      "to": "2018-01-10"
  }
  </code></pre>
  
  <p>Is it possible to map the instance attribute to another key (t_from -> from) when serializing?</p>