fix: Correct serializer model reference.

When linking a DRF serializer with a model, you need to link it to the
model class not an instance of the model.

The newer version of DRF tries to access the model_manager from the
model here and runs into issues if it's not defined correctly.

openedx/core/djangoapps/agreements/serializers.py

@@ -16,7 +16,7 @@ class IntegritySignatureSerializer(serializers.ModelSerializer):
     created_at = serializers.DateTimeField(source='created')
 
     class Meta:
-        model = IntegritySignature()
+        model = IntegritySignature
         fields = ('username', 'course_id', 'created_at')