title: __init__() got an unexpected keyword argument 'required' - Rest framework model serializer
body: <p>This is my code</p>
  
  <pre><code>from rest_framework import serializers
  from django.contrib.auth import get_user_model
  User = get_user_model()
  
  class UserSerializer(serializers.ModelSerializer):
      username = serializers.Field(source="username", required = False)   
      class Meta:
          model = User
          fields = ('first_name', 'last_name', 'username')
  </code></pre>
  
  <p>It seems so straight forward. What is the issue?</p>