A fix for Flask 2.0's async support.

@app.get('/')
async def say_hello():
    await asyncio.sleep(1)
    return {'message': 'Hello!'}


@app.get('/pets/<int:pet_id>')
@output(PetOutSchema)
async def get_pet(pet_id):
    if pet_id > len(pets) - 1:
        abort(404)
    return pets[pet_id]

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue and your username.
• Add *Version Changed* or *Version Added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.