SerializerMethodField can no longer be hinted with Optional types

[chaosk]

Hello!

It seems that a recent change in 753be1a made it so SerializerMethodField methods can no longer be annotated with typing.Optional.

Used versions:

• python 3.7.2
• django 2.2.2
• djangorestframework 3.9.4
• drf-yasg 1.16.0

Example code:

class PetSerializer(serializers.ModelSerializer):
    human_age = serializers.SerializerMethodField()

    def get_human_age(self, obj) -> Optional[int]:
        if obj.human_age_multiplier is None:
            return None
        return math.floor(obj.age * obj.human_age_multiplier)

    class Meta:
        model = Pet

Expected result:

human_age:
  title: Human age
  type: integer
  readOnly: true
  x-nullable: true

Actual result:

human_age:
  title: Human age
  type: string
  readOnly: true

[axnsan12]

Should be fixed by merged #428