Update API vies and serializers for the latest Django REST framework version. Replace DRF Swagger with DRF-YASG.

Signed-off-by: Michael Price <loneviking72@gmail.com>

mayan/apps/checkouts/serializers.py

@@ -2,18 +2,16 @@ from __future__ import unicode_literals
 
 from rest_framework import serializers
 
+from documents.serializers import DocumentSerializer
+
 from .models import DocumentCheckout
 
 
 class DocumentCheckoutSerializer(serializers.ModelSerializer):
-    def __init__(self, *args, **kwargs):
-        # Hide this import otherwise strange circular import error occur
-        from documents.serializers import DocumentSerializer
-
-        super(DocumentCheckoutSerializer, self).__init__(*args, **kwargs)
-        self.fields['document'] = DocumentSerializer()
+    document = DocumentSerializer()
 
     class Meta:
+        fields = ('document',)
         model = DocumentCheckout