[Bug] Inheritance does not work

Describe the bug According to the documentation, inheritance is supported. However, using the example code provided in the documentation, the expected structure containing members from both the base class and the subclass does not occur. Instead, only the members from the subclass are shown.

Screenshots

To Reproduce Use example code that is provided in the documentation:

from sanic import Sanic
from sanic.response import json

from sanic_openapi import doc, swagger_blueprint

app = Sanic()
app.blueprint(swagger_blueprint)

class User:
    username = doc.String("The name of your user account.")
    password = doc.String("The password of your user account.")

class UserInfo(User):
    first_name = doc.String("The first name of user.")
    last_name = doc.String("The last name of user.")

@app.get("/test")
@doc.produces(UserInfo)
async def test(request):
    return json({})

app.run(host="0.0.0.0", debug=True)

Expected behavior UserInfo on the UI page should contain "username" and "password" members in addition to "first_name" and "last_name".

Environment (please complete the following information):

OS: Windows 10
Browser Chrome

Package versions:

sanic==20.3.0
sanic-openapi==0.6.0

Additional context Add any other context about the problem here. This might be how to fix this bug or some hint to fix it.

sanic-org / sanic-openapi

[Bug] Inheritance does not work #177