swagger UI breaks if using openAPI 3.0.0

I've got a project which works perfectly at openapi version "2.0".
Changing to openAPI "3.0.0" causes an error-

```
TypeError: Object of type 'MyCoolMashmallowSchema' is not JSON serializable
```
this is my app configuration-
```
app.config.update(
{
"APISPEC_SPEC": APISpec(
title="Neptune Payload Manager",
version=app_version,
openapi_version='3.0.0',
plugins=[MarshmallowPlugin()],
),
"APISPEC_SWAGGER_UI_URL": "/",
"APISPEC_SWAGGER_URL": "/json",
}
)
```

this is my schema-
```
class MyCoolMashmallowSchema(Schema):

class Meta:
strict = True

name = fields.Str()
description = fields.Str()

class MyCoolObject:

# Not strictly necessary, but convenient to be able to access the schema this way
schema = MyCoolMarshmallowSchema

def __init__(
self,
name: str,
description: str,
):
self.name = name
self.description = description

@post_load
def make_model(self, data):

return MyCoolObject(
data["name"],
data["description"],
)
```
and my resource

```
@doc(
tags=['My Cool Object']
)
@marshal_with(MyCoolObject.schema(many=True), code=200, description='OK')
class MyCoolObjectListResource(MethodResource):

store: List[MyCoolObject] = [
MyCoolObject('test1', 'this is a made up object'),
MyCoolObject('test2', 'this is also a made up object'),
MyCoolObject('test3', 'this is also also a made up object'),
]

def get(self):
return self.store
```

anything obvious jump out?

Reproduce the failure using the APISpec configuration with openapi_version='3.0.0', then inspect the /json endpoint and Swagger UI response for the non-serializable schema. Compare the result with openapi_version='2.0'; done means the OpenAPI 3.0 specification is JSON-serializable and the UI loads successfully.

Written by the indexing model from the issue text.